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rfc:rfc3966

Network Working Group H. Schulzrinne Request for Comments: 3966 Columbia University Obsoletes: 2806 December 2004 Category: Standards Track

                 The tel URI for Telephone Numbers

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 This document specifies the URI (Uniform Resource Identifier) scheme
 "tel".  The "tel" URI describes resources identified by telephone
 numbers.  This document obsoletes RFC 2806.

Table of Contents

 1.   Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.   Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  4
 3.   URI Syntax. . . . . . . . . . . . . . . . . . . . . . . . . .  4
 4.   URI Comparisons . . . . . . . . . . . . . . . . . . . . . . .  6
 5.   Phone Numbers and Their Context . . . . . . . . . . . . . . .  6
      5.1.   Phone Numbers. . . . . . . . . . . . . . . . . . . . .  6
             5.1.1. Separators in Phone Numbers . . . . . . . . . .  7
             5.1.2. Alphabetic Characters Corresponding to Digits .  7
             5.1.3. Alphabetic, *, and # Characters as Identifiers.  7
             5.1.4. Global Numbers. . . . . . . . . . . . . . . . .  7
             5.1.5. Local Numbers . . . . . . . . . . . . . . . . .  8
      5.2.   ISDN Subaddresses. . . . . . . . . . . . . . . . . . .  9
      5.3.   Phone Extensions . . . . . . . . . . . . . . . . . . . 10
      5.4.   Other Parameters . . . . . . . . . . . . . . . . . . . 10
 6.   Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
 7.   Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 11
      7.1.   Why Not Just Put Telephone Numbers in SIP URIs?. . . . 11
      7.2.   Why Not Distinguish between Call Types?. . . . . . . . 11
      7.3.   Why tel. . . . . . . . . . . . . . . . . . . . . . . . 11
      7.4.   Do Not Confuse Numbers with How They Are Dialed. . . . 11

Schulzrinne Standards Track [Page 1] RFC 3966 The tel URI December 2004

 8.   Usage of Telephone URIs in HTML . . . . . . . . . . . . . . . 11
 9.   Use of "tel" URIs with SIP (Informative). . . . . . . . . . . 12
 10.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
 11.  Security Considerations . . . . . . . . . . . . . . . . . . . 14
 12.  Changes Since RFC 2806. . . . . . . . . . . . . . . . . . . . 14
 13.  References. . . . . . . . . . . . . . . . . . . . . . . . . . 15
      13.1.  Normative References . . . . . . . . . . . . . . . . . 15
      13.2.  Informative References . . . . . . . . . . . . . . . . 16
 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

 This document defines the URI scheme "tel", which describes resources
 identified by telephone numbers.  A telephone number is a string of
 decimal digits that uniquely indicates the network termination point.
 The number contains the information necessary to route the call to
 this point.  (This definition is derived from [E.164] but encompasses
 both public and private numbers.)
 The termination point of the "tel" URI telephone number is not
 restricted.  It can be in the public telephone network, a private
 telephone network, or the Internet.  It can be fixed or wireless and
 address a fixed wired, mobile, or nomadic terminal.  The terminal
 addressed can support any electronic communication service (ECS),
 including voice, data, and fax.  The URI can refer to resources
 identified by a telephone number, including but not limited to
 originators or targets of a telephone call.
 The "tel" URI is a globally unique identifier ("name") only; it does
 not describe the steps necessary to reach a particular number and
 does not imply dialling semantics.  Furthermore, it does not refer to
 a specific physical device, only to a telephone number.
 As commonly understood, telephone numbers comprise two related but
 distinct concepts: a canonical address-of-record and a dial string.
 We define the concepts below:
 Address-of-record or identifier: The telephone number is understood
    here as the canonical address-of-record or identifier for a
    termination point within a specific network.  For the public
    network, these numbers follow the rules in E.164 [E.164], while
    private numbers follow the rules of the owner of the private
    numbering plan.  Subscribers publish these identifiers so that
    they can be reached, regardless of the location of the caller.
    (Naturally, not all numbers are reachable from everywhere, for a

Schulzrinne Standards Track [Page 2] RFC 3966 The tel URI December 2004

    variety of technical and local policy reasons.  Also, a single
    termination point may be reachable from different networks and may
    have multiple identifiers.)
 Dial string: "Dial strings" are the actual numbers, symbols, and
    pauses entered by a user to place a phone call.  A dial string is
    consumed by one or more network entities and understood in the
    context of the configuration of these entities.  It is used to
    generate an address-of-record or identifier (in the sense
    described above) so that a call can be routed.  Dial strings may
    require prepended digits to exit the private branch exchange (PBX)
    the end system is connected to, and they may include post-dial
    dual-tone multi-frequency (DTMF) signaling that could control an
    interactive voice response (IVR) system or reach an extension.
    Dial strings are beyond the scope of this document.
 Both approaches can be expressed as a URI.  For dial strings, this
 URI is passed to an entity that can reproduce the actions specified
 in the dial string.  For example, in an analog phone system, a dialer
 translates the dial string into a sequence of actions such as waiting
 for dial tone, sending DTMF digits, pausing, and generating post-dial
 DTMF digits after the callee picks up.  In an integrated services
 digital network (ISDN) or ISDN user part (ISUP) environment, the
 signaling elements that receive protocol messages containing the dial
 string perform the appropriate protocol actions.  As noted, this
 approach is beyond the scope of this specification.
 The approach described here has the URI specify the telephone number
 as an identifier, which can be either globally unique or only valid
 within a local context.  The dialling application is aware of the
 local context, knowing, for example, whether special digits need to
 be dialed to seize an outside line; whether network, pulse, or tone
 dialling is needed; and what tones indicate call progress.  The
 dialling application then converts the telephone number into a dial
 sequence and performs the necessary signaling actions.  The dialer
 does not have to be a user application as found in traditional
 desktop operating systems but could well be part of an IP-to-PSTN
 gateway.
 To reach a telephone number from a phone on a PBX, for example, the
 user of that phone has to know how to convert the telephone number
 identifier into a dial string appropriate for that phone.  The
 telephone number itself does not convey what needs to be done for a
 particular terminal.  Instructions may include dialling "9" before
 placing a call or prepending "00" to reach a number in a foreign
 country.  The phone may also need to strip area and country codes.

Schulzrinne Standards Track [Page 3] RFC 3966 The tel URI December 2004

 The identifier approach described in this document has the
 disadvantage that certain services, such as electronic banking or
 voicemail, cannot be specified in a "tel" URI.
 The notation for phone numbers in this document is similar to that in
 RFC 3191 [RFC3191] and RFC 3192 [RFC3192].  However, the syntax
 differs as this document describes URIs whereas RFC 3191 and RFC 3192
 specify electronic mail addresses.  RFC 3191 and RFC 3192 use "/" to
 indicate parameters (qualifiers).  Since URIs use the forward slash
 to describe path hierarchy, the URI scheme described here uses the
 semicolon, in keeping with Session Initiation Protocol (SIP) URI
 conventions [RFC3261].
 The "tel" URI can be used as a request URI in SIP [RFC3261] requests.
 The SIP specification also inherits the 'subscriber' part of the
 syntax as part of the 'user element' in the SIP URI.  Other protocols
 may also use this URI scheme.
 The "tel" URI does not specify the call type, such as voice, fax, or
 data call, and does not provide the connection parameters for a data
 call.  The type and parameters are assumed to be negotiated either
 in-band by the telephone device or through a signaling protocol such
 as SIP.
 This document obsoletes RFC 2806.

2. Terminology

 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC
 2119, [RFC2119] and indicate requirement levels for compliant
 implementations.

3. URI Syntax

 The URI is defined using the ABNF (augmented Backus-Naur form)
 described in RFC 2234 [RFC2234] and uses elements from the core
 definitions (appendix A of RFC 2234).
 The syntax definition follows RFC 2396 [RFC2396], indicating the
 actual characters contained in the URI.  If the reserved characters
 "+", ";", "=", and "?" are used as delimiters between components of
 the "tel" URI, they MUST NOT be percent encoded.  These characters
 MUST be percent encoded if they appear in tel URI parameter values.

Schulzrinne Standards Track [Page 4] RFC 3966 The tel URI December 2004

 Characters other than those in the "reserved" and "unsafe" sets (see
 RFC 2396 [RFC2396]) are equivalent to their "% HEX HEX" percent
 encoding.
 The "tel" URI has the following syntax:
 telephone-uri        = "tel:" telephone-subscriber
 telephone-subscriber = global-number / local-number
 global-number        = global-number-digits *par
 local-number         = local-number-digits *par context *par
 par                  = parameter / extension / isdn-subaddress
 isdn-subaddress      = ";isub=" 1*uric
 extension            = ";ext=" 1*phonedigit
 context              = ";phone-context=" descriptor
 descriptor           = domainname / global-number-digits
 global-number-digits = "+" *phonedigit DIGIT *phonedigit
 local-number-digits  =
    *phonedigit-hex (HEXDIG / "*" / "#")*phonedigit-hex
 domainname           = *( domainlabel "." ) toplabel [ "." ]
 domainlabel          = alphanum
                        / alphanum *( alphanum / "-" ) alphanum
 toplabel             = ALPHA / ALPHA *( alphanum / "-" ) alphanum
 parameter            = ";" pname ["=" pvalue ]
 pname                = 1*( alphanum / "-" )
 pvalue               = 1*paramchar
 paramchar            = param-unreserved / unreserved / pct-encoded
 unreserved           = alphanum / mark
 mark                 = "-" / "_" / "." / "!" / "~" / "*" /
                        "'" / "(" / ")"
 pct-encoded          = "%" HEXDIG HEXDIG
 param-unreserved     = "[" / "]" / "/" / ":" / "&" / "+" / "$"
 phonedigit           = DIGIT / [ visual-separator ]
 phonedigit-hex       = HEXDIG / "*" / "#" / [ visual-separator ]
 visual-separator     = "-" / "." / "(" / ")"
 alphanum             = ALPHA / DIGIT
 reserved             = ";" / "/" / "?" / ":" / "@" / "&" /
                        "=" / "+" / "$" / ","
 uric                 = reserved / unreserved / pct-encoded
 Each parameter name ("pname"), the ISDN subaddress, the 'extension',
 and the 'context' MUST NOT appear more than once.  The 'isdn-
 subaddress' or 'extension' MUST appear first, if present, followed by
 the 'context' parameter, if present, followed by any other parameters
 in lexicographical order.
    This simplifies comparison when the "tel" URI is compared
    character by character, such as in SIP URIs [RFC3261].

Schulzrinne Standards Track [Page 5] RFC 3966 The tel URI December 2004

4. URI Comparisons

 Two "tel" URIs are equivalent according to the following rules:
 o  Both must be either a 'local-number' or a 'global-number', i.e.,
    start with a '+'.
 o  The 'global-number-digits' and the 'local-number-digits' must be
    equal, after removing all visual separators.
 o  For mandatory additional parameters (section 5.4) and the 'phone-
    context' and 'extension' parameters defined in this document, the
    'phone-context' parameter value is compared as a host name if it
    is a 'domainname' or digit by digit if it is 'global-number-
    digits'.  The latter is compared after removing all 'visual-
    separator' characters.
 o  Parameters are compared according to 'pname', regardless of the
    order they appeared in the URI.  If one URI has a parameter name
    not found in the other, the two URIs are not equal.
 o  URI comparisons are case-insensitive.
 All parameter names and values SHOULD use lower-case characters, as
 tel URIs may be used within contexts where comparisons are case
 sensitive.
 Section 19.1.4 in the SIP specification [RFC3261] discusses one such
 case.

5. Phone Numbers and Their Context

5.1. Phone Numbers

 The 'telephone-subscriber' part of the URI indicates the number.  The
 phone number can be represented in either global (E.164) or local
 notation.  All phone numbers MUST use the global form unless they
 cannot be represented as such.  Numbers from private numbering plans,
 emergency ("911", "112"), and some directory-assistance numbers
 (e.g., "411") and other "service codes" (numbers of the form N11 in
 the United States) cannot be represented in global (E.164) form and
 need to be represented as a local number with a context.  Local
 numbers MUST be tagged with a 'phone-context' (section 5.1.5).
 Implementations MUST NOT assume that telephone numbers have a
 maximum, minimum, or fixed length, or that they always begin with or
 contain certain digits.

Schulzrinne Standards Track [Page 6] RFC 3966 The tel URI December 2004

5.1.1. Separators in Phone Numbers

 Phone numbers MAY contain visual separators.  Visual separators
 ('visual-separator') merely aid readability and are not used for URI
 comparison or placing a call.
 Although it complicates comparisons, this specification retains
 visual separators in order to follow the spirit of RFC 2396
 [RFC2396], which remarks that "A URI often needs to be remembered by
 people, and it is easier for people to remember a URI when it
 consists of meaningful components".  Also, ISBN URNs documented in
 RFC 3187 [RFC3187] use visual separators in a manner similar to this
 specification.
 However, even though ITU-T E.123 [E.123] recommends the use of space
 characters as visual separators in printed telephone numbers, "tel"
 URIs MUST NOT use spaces in visual separators to avoid excessive
 escaping.

5.1.2. Alphabetic Characters Corresponding to Digits

 In some countries, it is common to write phone numbers with
 alphabetic characters corresponding to certain numbers on the
 telephone keypad.  The URI format does not support this notation, as
 the mapping from alphabetic characters to digits is not completely
 uniform internationally, although there are standards [E.161][T1.703]
 addressing this issue.

5.1.3. Alphabetic, *, and # Characters as Identifiers

 As called and calling terminal numbers (TNs) are encoded in BCD in
 ISUP, six additional values per digit can be encoded, sometimes
 represented as the hexadecimal characters A through F.  Similarly,
 DTMF allows for the encoding of the symbols *, #, and A through D.
 However, in accordance with E.164, these may not be included in
 global numbers.  Their meaning in local numbers is not defined here,
 but they are not prohibited.

5.1.4. Global Numbers

 Globally unique numbers are identified by the leading "+" character.
 Global numbers MUST be composed with the country (CC) and national
 (NSN) numbers as specified in E.123 [E.123] and E.164 [E.164].
 Globally unique numbers are unambiguous everywhere in the world and
 SHOULD be used.

Schulzrinne Standards Track [Page 7] RFC 3966 The tel URI December 2004

5.1.5. Local Numbers

 Local numbers are unique only within a certain geographical area or a
 certain part of the telephone network, e.g., a private branch
 exchange (PBX), a state or province, a particular local exchange
 carrier, or a particular country.  URIs with local phone numbers
 should only appear in environments where all local entities can
 successfully set up the call by passing the number to the dialling
 software.  Digits needed for accessing an outside line, for example,
 are not included in local numbers.  Local numbers SHOULD NOT be used
 unless there is no way to represent the number as a global number.
 Local numbers SHOULD NOT be used for several reasons.  Local numbers
 require that the originator and recipient are configured
 appropriately so that they can insert and recognize the correct
 context descriptors.  Since there is no algorithm to pick the same
 descriptor independently, labelling numbers with their context
 increases the chances of misconfiguration so that valid identifiers
 are rejected by mistake.  The algorithm to select descriptors was
 chosen so that accidental collisions would be rare, but they cannot
 be ruled out.
 Local numbers MUST have a 'phone-context' parameter that identifies
 the scope of their validity.  The parameter MUST be chosen to
 identify the local context within which the number is unique
 unambiguously.  Thus, the combination of the descriptor in the
 'phone-context' parameter and local number is again globally unique.
 The parameter value is defined by the assignee of the local number.
 It does NOT indicate a prefix that turns the local number into a
 global (E.164) number.
 There are two ways to label the context:  via a global number or any
 number of its leading digits (e.g., "+33") and via a domain name,
 e.g., "houston.example.com".  The choice between the two is left to
 the "owner" of the local number and is governed by whether there is a
 global number or domain name that is a valid identifier for a
 particular local number.
 The domain name does not have to resolve to any actual host but MUST
 be under the administrative control of the entity managing the local
 phone context.
 A global number context consists of the initial digits of a valid
 global number.  All global numbers with these initial digits must be
 assigned to the same organization, and no such matching number can be
 used by any other organization.  For example, +49-6151-16 would be a
 suitable context for the Technical University of Darmstadt, as it
 uses all numbers starting with those digits.  If such an initial

Schulzrinne Standards Track [Page 8] RFC 3966 The tel URI December 2004

 string of digits does not exist, the organization SHOULD use the
 lowest number of the global number range assigned to it.  (This can
 occur if two organizations share the same decimal block of numbers.
 For example, assume an organization owns the number range +1-212-
 555-0100 through +1-212-555-0149.  +1-212-555-1 would not be a valid
 global number context, but +1-212-555-0100 would work.) It is not
 required that local numbers within the context actually begin with
 the chosen set of initial numbers.
 A context consisting of the initial digits of a global number does
 not imply that adding these to the local number will generate a valid
 E.164 number.  It might do so by coincidence, but this cannot be
 relied upon.  (For example, "911" should be labeled with the context
 "+1", but "+1-911" is not a valid E.164 number.)
 National freephone numbers do not need a context, even though they
 are not necessarily reachable from outside a particular country code
 or numbering plan.  Recall that "tel" URIs are identifiers; it is
 sufficient that a global number is unique, but it is not required
 that it be reachable from everywhere.
    Even non-freephone numbers may be out of date or may not be
    reachable from a particular location.  For example, premium
    services such as "900" numbers in the North American numbering
    plan are often not dialable from outside the particular country
    code.
    The two label types were chosen so that, in almost all cases, a
    local administrator can pick an identifier that is reasonably
    descriptive and does not require a new IANA-managed assigned
    number.  It is up to the administrator to assign an appropriate
    identifier and to use it consistently.  Often, an organization can
    choose among several different identifiers.
 If the recipient of a "tel" URI uses it simply for identification,
 the receiver does not need to know anything about the context
 descriptor.  It simply treats it as one part of a globally unique
 identifier, with the other being the local number.  If a recipient of
 the URI intends to place a call to the local number, it MUST
 understand the context and be able to place calls within that
 context.

5.2. ISDN Subaddresses

 A phone number MAY also contain an 'isdn-subaddress' parameter that
 indicates an ISDN subaddress.

Schulzrinne Standards Track [Page 9] RFC 3966 The tel URI December 2004

 ISDN subaddresses typically contain International Alphabet 5 (IA5
 [T.50]) characters but may contain any octet value.

5.3. Phone Extensions

 Phone extensions identify stations behind a non-ISDN PBX and are
 functionally roughly equivalent to ISDN subaddresses.  They are
 identified with the 'extension' parameter.  At most, one of the
 'isdn-subaddress' and 'extension' parameters can appear in a "tel"
 URI, i.e., they cannot appear both at the same time.

5.4. Other Parameters

 Future protocol extensions to this URI scheme may add other
 parameters ('parameter' in the ABNF).  Such parameters can be either
 mandatory or optional.  Mandatory parameters start with "m-".  An
 implementation MAY ignore optional parameters and MUST NOT use the
 URI if it contains unknown mandatory parameters.  The "m-" prefix
 cannot be added to parameters that were already registered (except to
 create a new, logically distinct parameter).  The "phone-context"
 parameter in this document is mandatory,  and "isub" and "ext" are
 optional.
 New mandatory parameters must be described in a standards-track RFC,
 but an informational RFC is sufficient for optional parameters.
 For example, 'parameter' parameters can be used to store
 application-specific additional data about the phone number, its
 intended use, or any conversions that have been applied to the
 number.
 Entities that forward protocol requests containing "tel" URIs with
 optional parameters MUST NOT delete or modify parameters they do not
 understand.

6. Examples

 tel:+1-201-555-0123: This URI points to a phone number in the United
    States.  The hyphens are included to make the number more human
    readable; they separate country, area code and subscriber number.
 tel:7042;phone-context=example.com: The URI describes a local phone
       number valid within the context "example.com".
 tel:863-1234;phone-context=+1-914-555: The URI describes a local
    phone number that is valid within a particular phone prefix.

Schulzrinne Standards Track [Page 10] RFC 3966 The tel URI December 2004

7. Rationale

7.1. Why Not Just Put Telephone Numbers in SIP URIs?

 The "tel" URI describes a service, reaching a telephone number, that
 is independent of the means of doing so, be it via a SIP-to-PSTN
 gateway, a direct SIP call via E.164 number ("ENUM") translation
 [RFC3761], some other signaling protocols such as H.323, or a
 traditional circuit-switched call initiated on the client side via,
 say, the Telephony Application Programming Interface (TAPI).  Thus,
 in spirit, it is closer to the URN schemes that also leave the
 resolution to an external mechanism.  The same "tel" URI may get
 translated to any number of other URIs in the process of setting up
 the call.

7.2. Why Not Distinguish between Call Types?

 Signaling protocols such as SIP allow negotiating the call type and
 parameters, making the very basic indication within the URI scheme
 moot.  Also, since the call type can change frequently, any such
 indication in a URI is likely to be out of date.  If such designation
 is desired for a device that directly places calls without a
 signaling protocol such as SIP, mechanisms such as the "type"
 attribute for the "A" element in HTML may be more appropriate.

7.3. Why "tel"?

 "tel" was chosen because it is widely recognized that none of the
 other suggestions appeared appropriate.  "Callto" was discarded
 because URI schemes locate a resource and do not specify an action to
 be taken.  "Telephone" and "phone" were considered too long and not
 easily recognized internationally.

7.4. Do Not Confuse Numbers with How They Are Dialed

 As an example, in many countries the E.164 number "+1-212-555-3141"
 will be dialed  as 00-1-212-555-3141, where the leading "00" is a
 prefix for international calls.  (In general, a "+" symbol in E.164
 indicates that an international prefix is required.)

8. Usage of Telephone URIs in HTML

 Links using the "tel" URI SHOULD enclose the telephone number so that
 users can easily predict the action taken when following the link
 Dial <a href="tel:+1-212-555-0101">+1-212-555-0101</a> for
 assistance.

Schulzrinne Standards Track [Page 11] RFC 3966 The tel URI December 2004

 instead of
 Dial <a href="tel:+1-212-555-0101">this number</a> for assistance.
 On a public HTML page, the telephone number in the URI SHOULD always
 be in the global form, even if the text of the link uses some local
 format:
 Telephone (if dialling in the United States):
   <a href="tel:+1-201-555-0111">(201) 555-0111</a>
 or even
 For having RFCs read aloud, call <a
 href="tel:+1-555-438-3732">1-555-IETF-RFC</a>.

9. Use of "tel" URIs with SIP (Informative)

 SIP can use the "tel" URI anywhere a URI is allowed, for example as a
 Request-URI, along with "sip" and "sips" URIs.  For brevity, we will
 imply "sips" URIs when talking about SIP URIs.  Both "tel" and SIP
 URIs can contain telephone numbers.  In SIP URIs, they appear as the
 user part, i.e., before the @ symbol (section 19.1.6 in [RFC3261]).
 Unless a SIP UA connects directly to a PSTN gateway, one of the SIP
 proxy servers has to translate the "tel" URI to a SIP URI, with the
 host part of that URI pointing to a gateway.  Typically, the outbound
 proxy server, as the first proxy server visited by a call request,
 performs this translation.  A proxy server can translate all "tel"
 URIs to the same SIP host name or select a different gateway for
 different "tel" prefixes, based, for example, on information learned
 from TRIP [RFC3219].  However, a proxy server could also delegate
 this translation task to any other proxy server, as proxy servers are
 free to apply whatever routing logic they desire.  For local numbers,
 the proxy MUST NOT translate "tel" URIs whose contexts it does not
 understand.
 As noted earlier, all phone numbers MUST use the global form unless
 they cannot be represented as such.  If the local-number format is
 used, it MUST be qualified by the 'phone-context' parameter.
 Effectively, the combination of local number and phone context makes
 the "tel" URI globally unique.
 Although web pages, vCard business cards, address books, and
 directories can easily contain global "tel" URIs, users on twelve-
 button (IP) phones cannot dial such numbers directly and are
 typically accustomed to dialling shorter strings, e.g., for PBX
 extensions or local numbers.  These so-called dial strings (section

Schulzrinne Standards Track [Page 12] RFC 3966 The tel URI December 2004

 1) are not directly represented by "tel" URIs, as noted.  We refer to
 the rules that govern the translation of dial strings into SIP and
 "tel" URIs, global or local, as the dial plan.  Currently,
 translations from dial strings to "tel" URIs have to take place in
 end systems.  Future efforts may provide means to carry dial strings
 in a SIP URI, for example, but no such mechanisms exist as of this
 writing.
 A SIP UA can use a dial plan to translate dial strings into SIP or
 "tel" URIs.  The dial plan can be manually configured or, preferably,
 downloaded as part of a device configuration mechanism.  (At this
 time, there is no standardized mechanism for this.)
 A mobile user can use at least two dial plans, namely the dial plan
 for the network that he is currently visiting and the dial plan for
 his home network.  Generally, dialed numbers meant to represent
 global numbers will look the same after the translation regardless of
 the dial plan, even if, say, the visited network uses '0' for
 dialling an 'outside' number and the user's home network uses '9', as
 long as the user is aware of the current dial plan.  However, local
 extensions without a direct global equivalent may well behave
 differently.  To avoid any ambiguity, the dial plan MUST insert a
 suitable 'phone-context' string when performing the translation.  If
 the 'phone-context' is a domain name, there are three cases:
 1.  The outbound proxy recognizes the domain name in the "tel" or SIP
     URI as its local context and can route the request to a gateway
     that understands the local number.
 2.  The outbound proxy does not use the same phone context but can
     route to a proxy that handles this phone context.  This routing
     can be done via a lookup table, or the domain name of the phone
     context might be set up to reflect the SIP domain name of a
     suitable proxy.  For example, a proxy may always route calls with
     "tel" URIs like
        tel:1234;phone-context=munich.example.com
     to the SIP proxy located at munich.example.com.  (Proxies
     receiving a tel URI with a context they do not understand are
     obligated to return a 404 (Not Found) status response so that an
     outbound proxy may decide to attempt such a heuristic.)
 3.  The outbound proxy does not recognize the phone context and
     cannot find the appropriate proxy for that phone context.  In
     that case, the translation fails, and the outbound proxy returns
     a 404 (Not Found) error response.

Schulzrinne Standards Track [Page 13] RFC 3966 The tel URI December 2004

10. Acknowledgments

 This document is derived from RFC 2806 [RFC2806], written by Antti
 Vaehae-Sipilae.  Mark Allman, Flemming Andreasen, Francois Audet,
 Lawrence Conroy, Cullen Jennings, Michael Hammer, Paul Kyzivat,
 Andrew Main, Xavier Marjou, Jon Peterson, Mike Pierce, Jonathan
 Rosenberg, and James Yu provided extensive comments.

11. Security Considerations

 The security considerations parallel those for the mailto URL
 [RFC2368].
 Web clients and similar tools MUST NOT use the "tel" URI to place
 telephone calls without the explicit consent of the user of that
 client.  Placing calls automatically without appropriate user
 confirmation may incur a number of risks, such as those described
 below:
 o  Calls may incur costs.
 o  The URI may be used to place malicious or annoying calls.
 o  A call will take the user's phone line off-hook, thus preventing
    its use.
 o  A call may reveal the user's possibly unlisted phone number to the
    remote host in the caller identification data and may allow the
    attacker to correlate the user's phone number with other
    information, such as an e-mail or IP address.
 This is particularly important for "tel" URIs embedded in HTML links,
 as a malicious party may hide the true nature of the URI in the link
 text, as in
 <a href="tel:+1-900-555-0191">Find free information here</a>
 <a href="tel:+1-900-555-0191">tel:+1-800-555-0191</a>
 "tel" URIs may reveal private information, similar to including phone
 numbers as text.  However, the presence of the tel: schema identifier
 may make it easier for an adversary using a search engine to discover
 these numbers.

12. Changes Since RFC 2806

 The specification is syntactically backwards-compatible with the
 "tel" URI defined in RFC 2806 [RFC2806] but has been completely
 rewritten.  This document more clearly distinguishes telephone
 numbers as identifiers of network termination points from dial
 strings and removes the latter from the purview of "tel" URIs.

Schulzrinne Standards Track [Page 14] RFC 3966 The tel URI December 2004

 Compared to RFC 2806, references to carrier selection, dial context,
 fax and modem URIs, post-dial strings, and pause characters have been
 removed.  The URI syntax now conforms to RFC 2396 [RFC2396].
 A section on using "tel" URIs in SIP was added.

13. References

13.1. Normative References

 [E.123]    International Telecommunications Union, "Notation for
            national and international telephone numbers,  e-mail
            addresses and web addresses", Recommendation E.123,
            February 2001.
 [E.161]    International Telecommunications Union, "Arrangement of
            digits, letters and symbols on telephones and other
            devices that can be used for gaining access to a telephone
            network", Recommendation E.161, May 1995.
 [E.164]    International Telecommunications Union, "The international
            public telecommunication numbering plan", Recommendation
            E.164, May 1997.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 2234, November 1997.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP:  Session Initiation Protocol", RFC 3261,
            June 2002.
 [T1.703]   ANSI, "Allocation of Letters to the Keys of Numeric
            Keypads for Telecommunications", Standard T1.703-1995
            (R1999), 1999.

Schulzrinne Standards Track [Page 15] RFC 3966 The tel URI December 2004

13.2. Informative References

 [RFC2368]  Hoffman, P., Masinter, L., and J. Zawinski, "The mailto
            URL scheme", RFC 2368, July 1998.
 [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifiers (URI): Generic Syntax", RFC 2396,
            August 1998.
 [RFC2806]  Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806,
            April 2000.
 [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
            Resource Identifiers (URI) Dynamic Delegation Discovery
            System (DDDS) Application (ENUM)", RFC 3761, April 2004.
 [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
            Book Numbers as Uniform Resource Names", RFC 3187, October
            2001.
 [RFC3191]  Allocchio, C., "Minimal GSTN address format in Internet
            Mail", RFC 3191, October 2001.
 [RFC3192]  Allocchio, C., "Minimal FAX address format in Internet
            Mail", RFC 3192, October 2001.
 [RFC3219]  Rosenberg, J., Salama, H., and M. Squire, "Telephony
            Routing over IP (TRIP)", RFC 3219, January 2002.
 [T.50]     International Telecommunications Union, "International
            Reference Alphabet (IRA) (Formerly International Alphabet
            No. 5 or IA5) - Information technology - 7-bit coded
            character set for information interchange", Recommendation
            T.50, 1992.

Author's Address

 Henning Schulzrinne
 Columbia University
 Department of Computer Science
 450 Computer Science Building
 New York, NY  10027
 US
 Phone: +1 212 939 7042
 EMail: hgs@cs.columbia.edu
 URI:   http://www.cs.columbia.edu

Schulzrinne Standards Track [Page 16] RFC 3966 The tel URI December 2004

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Schulzrinne Standards Track [Page 17]

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