Multiple Subscriber Number

Description

Multiple Subscriber Number (MSN) is a service capability that allows a single subscriber identity (linked to an IMSI or IMS Public User Identity) to be associated with more than one E.164 directory number (MSISDN). Architecturally, this involves coordination between the HSS/UDM, which stores the association of multiple MSISDNs to a single user profile, and the Telephony Application Server (TAS) or other service logic in the IMS core. When a call is placed to one of the MSNs, the network (e.g., the MSC Server for circuit-switched or the IMS core for IP-based calls) uses signaling information, like the Called Party Number, to identify the target MSISDN. It then queries the HSS/UDM to resolve this MSISDN to the corresponding subscriber's IMSI or IMPU. The key technical mechanism is the mapping and routing logic that treats each MSISDN as a separate addressable endpoint that ultimately resolves to the same physical or logical user equipment. Service logic on the TAS can be tailored per MSISDN; for instance, different call forwarding rules, voicemail greetings, or charging rates can be applied depending on which number was called. In the IMS context, each MSISDN is typically associated with a distinct Tel URI (e.g., tel:+1234567890), and these may be implicitly or explicitly registered as Public User Identities linked to the same private user identity and service profile. This allows the network to present the appropriate identity (the called number) to the called party and to apply service logic specific to that 'line' or identity.

Purpose & Motivation

The MSN service was developed to meet the demand for a single device to support multiple telephony identities for different roles or contexts, such as separate numbers for personal life, work, and fax line. Before such a standardized feature, users had to carry multiple devices or SIM cards. The problem it solves is identity management and service differentiation on a single subscription. It allows operators to offer flexible service packages where a user can have distinct billing, call routing, and supplementary services (like voicemail) for each number, all billed to a single account. Its creation was motivated by business user needs and the evolution towards converged services. It provides a network-centric solution for multi-identity management, which is more seamless than relying on device-side applications or multiple SIMs, and integrates cleanly with existing network routing and service control infrastructure.