C subscriber

Description

In mobile telephony, the C subscriber is the party to whom a call is forwarded when the B subscriber has activated a call forwarding service. The B subscriber is the original called party (the number dialed by the A subscriber, who is the calling party). If the B subscriber has set up call forwarding – for example, to a voicemail system, a colleague, or a home phone – the call is redirected to the C subscriber. The C subscriber can be any telephone number, whether fixed or mobile, and may belong to the same network or a different one. This concept is fundamental to call forwarding supplementary services, which are defined in 3GPP specifications such as 22.031 (Call Forwarding), 23.018 (Basic Call Handling), 23.031 (Call Forwarding supplementary services), and 23.079 (Optimal Routing).

The C subscriber is not a network element but a role in a call scenario. When a call is forwarded, the network must handle the redirection transparently, ensuring that the call is routed to the C subscriber without the A subscriber being aware of the forwarding (unless notifications are configured). The forwarding can be unconditional (all calls), conditional (e.g., when busy, not reachable, or no answer), or based on specific criteria. The C subscriber may also have their own services, such as call waiting or forwarding, which can lead to cascading forwards, though networks typically limit the number of forwards to prevent loops.

From an architectural perspective, call forwarding is implemented in the mobile network using the Home Location Register (HLR) or Home Subscriber Server (HSS) for subscription data, and the Mobile Switching Center (MSC) or Mobile Softswitch (MSS) for call control. When the B subscriber has forwarding active, the MSC handling the call queries the HLR/HSS for the forwarding number (the C subscriber's number). The call is then routed towards the C subscriber, possibly through an Gateway MSC (GMSC) if the C subscriber is in another network. The C subscriber's own network then delivers the call as normal.

The concept of the C subscriber is also relevant in the context of optimal routing (23.079), which aims to avoid unnecessary trombone connections when both the B and C subscribers are in the same network or when the call can be routed more efficiently. In such cases, the network may route the call directly from the originating MSC to the C subscriber's visited MSC, bypassing the HLR interrogation if possible. This improves efficiency and reduces latency.

In 3GPP specifications, the C subscriber is defined in the vocabulary (21.905) and in the call handling specifications. The term has been stable since Rel-4, with no changes to its definition across releases up to Rel-19. This stability reflects the maturity of call forwarding services and their continued importance in both circuit-switched and, by extension, IP Multimedia Subsystem (IMS) environments, where similar concepts apply (e.g., the forwarding destination in IMS is often called the 'forwarded-to' party).

Understanding the C subscriber is essential for telecom engineers because it clarifies the roles in call forwarding scenarios and helps in troubleshooting call routing issues. It also highlights the distinction between the original called party (B) and the final recipient (C), which is crucial for billing, lawful interception, and service logic.

Purpose & Motivation

The concept of the C subscriber exists to clearly define the final destination of a forwarded call in mobile networks. Call forwarding is a widely used supplementary service that allows subscribers to redirect incoming calls to another number when they are unavailable, busy, or simply prefer another device. Without a clear term for the forwarded-to party, discussions of call routing, billing, and service interactions would be ambiguous. The C subscriber provides that clarity, distinguishing the original called party (B) from the ultimate recipient (C).

Historically, as mobile networks evolved and call forwarding became a standard feature, 3GPP needed a consistent vocabulary to describe the various parties in a call. The A subscriber (caller), B subscriber (called party), and C subscriber (forwarded-to party) form a triad that is used throughout specifications. This terminology was introduced in early releases (Rel-4) and has remained unchanged because the fundamental concept of call forwarding has not changed. Even with the advent of VoLTE and IMS, the same roles apply, although the implementation may differ.

The C subscriber term also helps in defining the behavior of other services, such as call deflection, where the B subscriber can redirect an incoming call to a C subscriber before answering. It is also relevant for multi-party calls and call transfer scenarios. By standardizing this term, 3GPP ensures that network equipment from different vendors can interoperate correctly when handling forwarded calls, and that charging and accounting systems can correctly identify the parties involved.