Explicit Communication Transfer

Description

Explicit Communication Transfer (ECT) is a standardized supplementary service within the 3GPP framework that enables a served user (User A) to transfer an established communication (typically a speech call) to another party (User C). The process is 'explicit' because it involves active user invocation and in-call signaling, rather than an automatic or pre-defined diversion. Architecturally, ECT relies on the call control and session management functions in the core network, specifically in the MSC server for CS calls or the IMS core for IP-based calls. The key components involved are the transferring user's terminal, the network's service control logic, and the target user's terminal. The service operates through a defined procedure: User A, engaged in a call with User B, may place B on hold, establish a new call or consultation call to User C, and then explicitly issue a transfer request. The network then connects User B and User C directly, releasing User A from the connection. ECT can be implemented as a 'blind transfer' (immediate transfer without consultation) or a 'consultative transfer' (consultation with the third party first). Its role is to provide advanced call handling capabilities, facilitating features like call forwarding on busy or no answer in a more dynamic, user-controlled manner. The service is detailed across numerous 3GPP specifications covering CS core (TS 24.083) and IMS (TS 24.604), defining the protocols (like ISUP, SIP) and timers required for interoperable implementation.

Purpose & Motivation

ECT was developed to address the need for flexible, user-initiated call management beyond basic call forwarding settings configured in advance. Prior to its standardization, call transfer capabilities were often proprietary or limited to specific PBX systems, hindering interoperability across different network operators and terminal types. The creation of ECT provided a uniform, network-controlled mechanism that works across multi-vendor, multi-operator environments. It solved the problem of efficiently redirecting calls in professional and personal contexts—for example, transferring a customer call to a specialist or redirecting a call to a mobile phone when leaving an office. The historical context includes its definition in early GSM standards and its subsequent evolution into the IP Multimedia Subsystem (IMS), ensuring service continuity as networks migrated to all-IP architectures. ECT addresses limitations of implicit or network-centric transfers by giving the transferring user explicit control over the process.