Call Transfer supplementary service

Description

Call Transfer (CT) is a standardized supplementary service defined within the 3GPP Core Network and service architecture. It operates at the application layer, interacting with the Call Control (CC) entity within the circuit-switched (CS) domain, and is managed by the Home Location Register (HLR) and Mobile Switching Center (MSC) or IP Multimedia Subsystem (IMS) core for later releases. The service logic is executed in network nodes, not the User Equipment (UE), though the UE provides the user interface for activation and invocation.

The service works through a defined sequence of signaling messages between the involved parties: the transferring party (User A), the original called party (User B), and the transferred-to party (User C). When User A invokes CT during an active call with User B, the network places User B on hold and establishes a new call leg to User C. Depending on the transfer variant, User A may then either drop out of the call (completing the transfer) or conference all parties together. Key protocol components include the ISDN User Part (ISUP) or Bearer Independent Call Control (BICC) in the CS core, and Session Initiation Protocol (SIP) REFER methods within IMS.

Architecturally, the service is provisioned in the user's service profile stored in the HLR/Home Subscriber Server (HSS). During registration or call setup, this profile is downloaded to the serving MSC or Call Session Control Function (CSCF). The actual service execution is performed by the MSC Server or the Telephony Application Server (TAS) in an IMS environment. The service interacts with basic call state models to manage the multiple call legs and their associations.

Its role in the network is to enrich the basic telephony service offering, enabling sophisticated call-handling scenarios essential for business and personal use. It forms part of the suite of supplementary services like Call Forwarding (CF) and Call Waiting (CW), which together provide a complete feature-rich telephony experience. In modern networks, especially with VoLTE and VoNR, CT is implemented as an IMS-based service, ensuring seamless operation across access technologies.

Purpose & Motivation

CT was created to replicate and standardize the call transfer capabilities found in traditional landline Private Branch Exchange (PBX) and business telephone systems for the mobile domain. Prior to its standardization in 3GPP, mobile networks offered primarily basic call origination and termination. The lack of advanced call-handling features limited the utility of mobile phones for professional and collaborative use cases, where transferring a call to a more appropriate colleague or department is a common requirement.

The problem it solves is the static nature of a two-party call. In many situations, the person who answers a call may not be the correct endpoint for the conversation. CT dynamically re-routes an established call session to a third party without requiring the original called party to hang up and call again. This saves time, improves user experience, and enhances productivity by facilitating efficient call routing within organizations.

Historically, its introduction in Release 5 was part of a broader effort to make GSM/UMTS networks feature-competitive with fixed-line ISDN services. It addressed the limitation of earlier mobile systems that lacked standardized, network-centric supplementary services. By providing a uniform mechanism across different network operators and terminal vendors, CT ensured interoperability and a consistent user experience, which was crucial for the adoption of mobile phones as primary business tools.

Key Features

• Enables transfer of an active call to a third party
• Supports variants like Explicit Call Transfer (ECT) where the transferor explicitly initiates the action
• Network-based service execution, independent of terminal capabilities
• Interworks with basic call control and other supplementary services
• Provisioning and subscription controlled via HLR/HSS
• Standardized signaling procedures for reliable operation across vendor equipment

Evolution Across Releases

Defining Specifications