B Subscriber (Called Party)

Description

The B subscriber is a fundamental service-layer concept in 3GPP specifications that identifies the called party in a mobile-originated call or session. In a typical mobile call, the A subscriber is the calling party and the B subscriber is the party being called. The B subscriber may be a mobile subscriber, a fixed-line subscriber, or an application server, depending on the service context. From a service perspective, the B subscriber is the final destination of the call attempt, and the network must locate, route to, and deliver the call to that subscriber while respecting the B subscriber's subscription profile, supplementary services, and operator policies.

In circuit-switched (CS) networks based on 3GPP TS 23.018 and related specifications, the B subscriber is identified by an MSISDN or an IMSI. The Mobile Switching Center (MSC) handling the call queries the Home Location Register (HLR) for the B subscriber's routing information (e.g., Mobile Station Roaming Number, MSRN). The HLR then interacts with the Visitor Location Register (VLR) serving the B subscriber to obtain the MSRN, which is used by the call-control function to route the call to the visited MSC. The B subscriber's subscription data, such as call forwarding, call barring, and call waiting, is evaluated before the call is delivered.

In IP Multimedia Subsystem (IMS) and VoLTE deployments, the B subscriber is identified by a SIP URI or a Tel URI. The originating Serving-CSCF (S-CSCF) evaluates the A subscriber's originating services and then routes the session to the terminating S-CSCF of the B subscriber. The terminating S-CSCF applies the B subscriber's terminating services, such as call forwarding, call barring, and communication diversion, before terminating the session to the B subscriber's UE. The B subscriber's registration state and service profile are managed by the Home Subscriber Server (HSS).

The concept is widely used in charging and billing. Charging records distinguish between the A subscriber and the B subscriber to allow separate charging for originating and terminating parties. In interconnect scenarios, the B subscriber's network may charge the A subscriber's network for termination, and settlement between operators depends on correct identification of the B subscriber and the corresponding network.

The B subscriber also plays a role in supplementary services. For example, in Call Forwarding Unconditional (CFU), the B subscriber's profile may redirect the call to a forwarded-to number. In Multi-party or Conference services, multiple B subscribers may be involved. In emergency calls, the concept is adjusted because emergency calls are not addressed to a specific B subscriber but to a Public Safety Answering Point (PSAP), which acts as the terminating party.

In 5G and IMS frameworks, the B subscriber concept persists for voice and multimedia telephony services, including 5G New Radio (NR) voice over IMS. The term is also used in lawful interception, where the B subscriber may be the target of interception if authorized. Overall, the B subscriber is a service-layer abstraction that enables the network to correctly route, authorize, and charge for terminating calls and sessions.

Purpose & Motivation

The B subscriber concept exists to provide a precise, standardized way to refer to the called party in telecommunication services. It separates the roles of calling and called parties, which is essential for routing, service control, charging, and supplementary service execution. Without a clear distinction, network elements would not know whose subscription profile to apply, where to route the call, or whom to charge.

Historically, early mobile networks such as NMT and GSM introduced the A and B subscriber notation to manage call delivery across visited and home networks. As services evolved from basic voice to supplementary services, IMS, and VoLTE, the B subscriber concept was extended to cover IP-based sessions and multimedia services. It solved the problem of inconsistent terminology across different network domains and allowed specifications to define interactions between network entities in a generic, technology-agnostic way.

The B subscriber also enables interworking between circuit-switched and packet-switched domains, and between different operators. In roaming scenarios, the B subscriber may be served by a visited network, and the home network must coordinate with the visited network to deliver the call. The B subscriber concept is therefore central to roaming, interconnect, and settlement processes.