Transfer To Third Party

Description

Transfer To Third Party (TTTP) is a Circuit-Switched (CS) supplementary service standardized by 3GPP. It allows a served user (User A), who is engaged in an active call with another party (User B), to transfer that call to a third party (User C). The service is invoked by User A, typically through a specific procedure like placing User B on hold, establishing a new call to User C, and then initiating the transfer. The network then connects User B and User C directly, releasing User A from the connection. This is distinct from call forwarding, which operates on incoming calls before they are answered; TTTP operates on an already established, active call.

The service architecture relies on the capabilities of the Mobile Switching Centre (MSC) and the Home Location Register (HLR) in the CS core network. The HLR stores the subscriber's service profile, indicating whether TTTP is provisioned. When a user initiates a transfer, the serving MSC executes the service logic. This involves managing multiple call legs: the original leg (A-B), the consultation leg (A-C), and finally establishing the new leg (B-C) while clearing the legs involving User A. The service uses standard ISDN User Part (ISUP) signaling or Bearer Independent Call Control (BICC) protocols to manage these connections between network nodes.

From a signaling perspective, the process involves specific messages and procedures defined in the core network specifications. The user typically provides the third-party number via Dual-Tone Multi-Frequency (DTMF) digits or a flash-hook event. The MSC interprets this as a service request, suspends the original call, and initiates the new call setup. Upon successful connection to the third party, the MSC performs a connect operation to join the two remote parties. A key aspect is ensuring proper charging; the service logic must correctly attribute the call segments to the appropriate parties, often involving specific charging triggers and records.

TTTP's role is to provide advanced call control, a feature carried over from fixed-line ISDN services into mobile networks. It is part of a suite of supplementary services that enrich basic telephony, supporting business and personal communication scenarios where call redirection is needed. While its usage has diminished with the rise of IP-based services and smartphone apps, it remains a standardized capability within the 3GPP CS domain, ensuring interoperability and consistent service behavior across different network operators and regions.

Purpose & Motivation

TTTP was created to bring advanced telephony features from the fixed ISDN world into the mobile domain, standardizing them for global interoperability. Prior to standardization, proprietary implementations or a lack of such features limited user control over active calls. The problem it solves is the need for a user to manually manage a three-way call scenario where the intent is to connect two other parties and then disconnect themselves—a common requirement in business environments for transferring calls to colleagues or departments.

Historically, without TTTP, a user would have to hang up and have one party call the other, or use a conference bridge feature and then drop out, which is less efficient and can be confusing. TTTP provides a standardized, network-controlled procedure that is reliable and billable. Its creation was motivated by the desire to offer a complete portfolio of ISDN-like supplementary services in GSM and UMTS networks, making mobile telephony as feature-rich as its fixed-line counterpart and meeting enterprise customer expectations.

The service addresses limitations in basic call handling by providing a formalized mechanism for call transfer that integrates seamlessly with network charging and security mechanisms. It ensures that the transfer is executed correctly by the network, maintaining call quality and providing appropriate signaling to all parties. While its relevance has waned in the era of VoLTE and OTT communication apps, which may implement similar functionality at the application layer, TTTP remains an important part of the legacy CS service framework defined by 3GPP.

Key Features

• Enables transfer of an established active call to a third-party number
• Network-based service executed by the MSC using HLR-subscribed profiles
• Uses standard ISUP/BICC signaling for inter-exchange call control
• Supports invocation via DTMF digits or flash-hook signals from the user
• Provides proper charging records for the transferring and transferred call segments
• Integrates with other supplementary services like call hold and call waiting

Evolution Across Releases

Defining Specifications