URI TELephone numbers Uniform Resource Identifier

Description

The TEL URI is a Uniform Resource Identifier scheme specifically designed for representing telephone numbers. Its syntax is defined by the IETF in RFC 3966 and is profiled and utilized within numerous 3GPP specifications, particularly those related to IP Multimedia Subsystem (IMS) and enriched communication services. A TEL URI begins with the scheme 'tel:' followed by a telephone number, which can be a global number (e.g., tel:+1-555-123-4567) or a local number with a context (e.g., tel:4567;phone-context=example.com). The '+' prefix indicates the number is globally unique according to the E.164 international numbering plan. The URI can also include parameters for extensions, isdn-subaddress, or service-specific contexts.

Within the 3GPP architecture, the TEL URI plays a crucial role in routing and identification. In IMS, a user's Public User Identity (IMPU) can be a SIP URI (e.g., sip:[email protected]) or a TEL URI (e.g., tel:+441234567890). This allows traditional telephone numbers to be used natively within the SIP-based IMS ecosystem. When a call is placed to a TEL URI, the IMS network performs number normalization and uses ENUM (E.164 Number Mapping) or other routing databases to translate the telephone number into a routable SIP URI (if the target is an IMS subscriber) or to break out to the circuit-switched network via a Media Gateway Control Function (MGCF).

The TEL URI is also extensively used in messaging services (e.g., SMS over IP, MMS), presence services, and supplementary service data (defined in specifications like 24.380 for UE configuration). It provides a standardized way for applications to reference a telephone number, ensuring interoperability between web services, email clients, contact applications, and network elements. Its definition includes rules for visual formatting and comparison (e.g., removing visual separators like hyphens and spaces for canonical comparison), which is vital for functions like click-to-dial and contact list matching.

Purpose & Motivation

The TEL URI was created to bridge the gap between the traditional telephone numbering world (E.164) and the Internet's URI-based addressing schemes. Before its adoption, representing a phone number in an IP context was ad-hoc, often using non-standard 'callto:' links or simply displaying the number as text, which applications could not reliably interpret for dialing purposes. The 'tel:' URI scheme provides a standardized, machine-readable format that allows user agents (like web browsers or email clients) to unambiguously identify a string of text as a telephone number and offer appropriate actions (e.g., initiate a voice call via VoIP or the native dialer).

3GPP's profiling of the TEL URI was motivated by the convergence of circuit-switched telephony and packet-switched IP services in networks like IMS. It solves the problem of identity management for users who primarily identify themselves with a phone number. By allowing a TEL URI to be a first-class Public User Identity in IMS, subscribers can use their existing mobile number for next-generation VoIP, video calling, and messaging services without needing an email-like SIP address. This was critical for user adoption and service interoperability, enabling seamless routing between legacy PSTN/PLMN networks and IMS domains. It addresses the limitation of having separate, incompatible addressing schemes for telephony and the internet.