IP Multimedia Subsystem Multimedia Telephony Service

Description

IMS Multimedia Telephony (MMTel) is a service defined by 3GPP that allows multimedia conversational communications between two or more users. It provides real-time bidirectional conversational transfer of media such as speech, video, text, or other types of data. The service includes Supplementary Services and takes account of regulatory requirements, making it the IMS equivalent of traditional circuit-switched telephony extended with multimedia capabilities. It is defined in 3GPP TS 21.905 (Vocabulary for 3GPP Specifications) and elaborated in the 22.173 series for service requirements.

MMTel is built on the IMS core network architecture. Session control is performed by the Call Session Control Functions (P-CSCF, I-CSCF, S-CSCF), while the MMTel application server (MMTel AS) hosts the service logic and supplementary services. The HSS provides subscriber profile data, including which supplementary services are provisioned. Media is transported using RTP over UDP/IP, with SIP used for session establishment, modification, and teardown. The service supports a wide range of codecs for speech and video, and can interwork with circuit-switched networks via media gateways and the IMS Centralized Services (ICS) and Service Centralization and Continuity (SCC) mechanisms.

Key capabilities include multi-party conferencing, media negotiation and renegotiation, and a rich set of supplementary services such as call forwarding, call barring, call waiting, and explicit call transfer. Because MMTel is an IMS application, it benefits from IMS security (authentication via ISIM/UICC, IPsec for SIP signaling), QoS control through the Policy and Charging Control (PCC) framework, and unified charging. It also supports regulatory requirements such as emergency calls and lawful interception, which are essential for an operator-grade telephony replacement.

MMTel is strategically important because it enables operators to migrate voice from circuit-switched networks to all-IP IMS while preserving the user experience and regulatory obligations. It is a cornerstone of VoLTE (Voice over LTE) and VoNR (Voice over New Radio), where MMTel provides the service layer on top of the IMS core. It also underpins Rich Communication Services (RCS) for enhanced messaging and call features.

The definition of IMS Multimedia Telephony in TS 21.905 has remained stable from Rel-9 through Rel-19. The quoted definitions across these releases are textually identical, indicating that the service concept and its scope were fully consolidated by Rel-9 and have not required revision in the vocabulary specification since.

Purpose & Motivation

IMS Multimedia Telephony was created to provide a standardized, operator-controlled replacement for circuit-switched telephony in an all-IP environment. As mobile networks evolved toward LTE and later 5G, circuit-switched voice could no longer be supported natively, so a service layer was needed to deliver voice and multimedia conversational services over IMS with equivalent quality, features, and regulatory compliance. MMTel fills this role by defining a complete conversational multimedia service that includes supplementary services and emergency call support.

Before MMTel, voice services were tied to circuit-switched infrastructure, which was inefficient for packet-based networks and limited the ability to offer integrated multimedia. Early VoIP solutions lacked standardized supplementary services, QoS integration, and regulatory features. MMTel addresses these gaps by leveraging IMS capabilities — SIP session control, PCC-based QoS, ISIM-based authentication, and standardized supplementary service definitions — to deliver a carrier-grade telephony experience.

MMTel also enables service continuity and centralization, allowing operators to offer a consistent service set across access technologies (LTE, 5G NR, Wi-Fi) and to interwork with legacy networks during migration. This makes it a key enabler for VoLTE, VoNR, and the long-term vision of a single, all-IP service core.