Correlation Mobile Station International Subscriber Directory Number

Description

The C-MSISDN is a critical identifier within the Voice Call Continuity (VCC) architecture defined in 3GPP Release 8 and later. It functions as a temporary, session-specific Mobile Station International Subscriber Directory Number assigned by the VCC Application in the IMS network when a UE initiates or receives a VCC-enabled call. This temporary number is distinct from the user's primary MSISDN and is used exclusively for the duration of the VCC session to facilitate domain transfer procedures.

Architecturally, the C-MSISDN operates within the VCC Application, which resides in the IMS core network. When a VCC-capable UE registers with the network, the VCC Application may pre-allocate a pool of C-MSISDNs or allocate them on-demand. During call setup, if the call is subject to VCC procedures, the VCC Application assigns a C-MSISDN to the UE and stores the mapping between this temporary identifier, the user's primary identity (such as IMS Public User Identity), and the current call session information. This mapping is maintained in the VCC Application's database throughout the call session.

The C-MSISDN enables the network to correlate the two legs of a VCC call: the Circuit-Switched leg (typically over GERAN/UTRAN) and the IMS leg (over E-UTRAN or other IP-CAN). When a domain transfer is initiated (for example, from IMS to CS), the VCC Application uses the C-MSISDN to identify the target UE in the CS domain. The network routes the CS call leg to this temporary number, which is recognized by the VCC Application. The application then bridges the incoming CS leg with the existing IMS leg, maintaining call continuity without requiring the called party to redial or re-establish the connection.

Key components involved in C-MSISDN handling include the VCC Application itself, the Home Subscriber Server (HSS) for subscriber data management, the MSC Server for CS domain control, and the Serving-Call Session Control Function (S-CSCF) for IMS session control. The C-MSISDN must be routable in both CS and IMS domains and typically follows the same numbering format as regular MSISDNs, though it is allocated from a dedicated number range managed by the operator.

From a protocol perspective, the C-MSISDN appears in various signaling messages including SIP messages within IMS (particularly in the P-Asserted-Identity header) and ISUP/BICC messages in the CS domain. The VCC Application ensures that the C-MSISDN is properly inserted into relevant signaling paths and that the correlation between domains is maintained throughout the call session, including during mid-call transfers and call termination procedures.

Purpose & Motivation

The C-MSISDN was created to solve the fundamental problem of maintaining voice call continuity when a user equipment moves between different access network technologies, specifically between Circuit-Switched (CS) domains (like 2G/3G networks) and IP Multimedia Subsystem (IMS) domains (like LTE/5G networks). Before VCC and the C-MSISDN mechanism, calls would typically drop when transitioning between these fundamentally different network architectures because there was no standardized way to correlate the two independent call legs that existed in separate domains.

Historically, as operators began deploying IMS-based voice services (VoLTE) alongside traditional CS voice networks, they needed a solution for seamless handover between these domains, particularly for areas with patchy LTE coverage. The C-MSISDN provides the essential correlation mechanism that enables Single Radio Voice Call Continuity (SRVCC) and other VCC variants to function properly. Without this temporary identifier, the network would have no way to associate an incoming CS call leg with an ongoing IMS session, making domain transfers impossible without call interruption.

The C-MSISDN addresses several specific limitations of previous approaches: it eliminates the need to expose the user's actual MSISDN during domain transfer procedures (enhancing privacy), provides a clean separation between user identity and session routing, and enables standardized interworking between CS and IMS networks from different vendors. By serving as a session-specific routing address, the C-MSISDN allows operators to implement VCC without requiring changes to existing CS network elements or breaking existing charging and lawful interception systems.