First-to-answer call

Description

A first-to-answer call is a specific call setup mode defined within the Mission Critical Push To Talk (MCPTT) service architecture specified in 3GPP TS 23.379 (functional architecture and information flows) and TS 24.379 (MCPTT call control protocol). It applies to group calls where a call originator wishes to reach a set of potential target recipients, but the call is only actually established once the first of those recipients answers. This is distinct from a normal prearranged group call, where the call is set up immediately across the group's affiliated members, and from a broadcast group call, where no answer is required at all.

The defining characteristic of a first-to-answer call is the mandatory manual answer. The MCPTT client of each potential target recipient presents the incoming call to the user, and the user must explicitly accept it. Automatic answer — a capability that MCPTT supports for certain other call types (for example, where an authorised user or an emergency condition permits it) — is explicitly disallowed for first-to-answer calls. This guarantees that the call is only created when a human recipient has deliberately chosen to participate.

From a signalling perspective, the call originator sends a call setup request toward the MCPTT server indicating the target group or list of users and the first-to-answer behaviour. The server distributes the call announcement to the potential recipients. Each recipient's client responds with an answer or reject indication. The first answer received by the server triggers the actual call establishment: the server creates the call session, notifies the originator, and brings the answering user into the call. Subsequent answers from other recipients join the already-established call, while recipients who reject or do not answer are simply not included.

This behaviour is particularly valuable in operational scenarios where the originator does not know in advance which team member is available, on shift, or closest to an incident. Rather than forcing the originator to poll individuals sequentially, the first-to-answer mechanism lets the originator issue a single request and let availability determine who responds. It also avoids the resource consumption and unwanted audio of setting up a full group call when only one responder is actually needed.

Because the call only exists after the first manual answer, the first-to-answer call has implications for floor control, media transport, and charging. Floor control procedures (as defined in TS 24.380) begin once the call is established. Media is only distributed to participating clients. The MCPTT server must manage the transition from the pre-answer announcement state to the established call state, including any necessary bearer setup for the answering user and the originator.

First-to-answer calls are one of several MCPTT call types, alongside prearranged group calls, chat group calls, broadcast group calls, private calls, and emergency calls. Together they give mission-critical users a flexible toolkit for matching communication behaviour to operational need. The first-to-answer call occupies a middle ground between the immediacy of a prearranged group call and the selectivity of a private call, and its manual-answer requirement is a deliberate design choice to prevent unintended call establishment.

Purpose & Motivation

The first-to-answer call exists to solve a practical operational problem in mission-critical communications: an originator often needs to reach whoever among a set of users is available first, without knowing in advance who that will be. Traditional prearranged group calls set up the entire group immediately, which can be wasteful when only one responder is needed, and can create unwanted audio for users who are not in a position to participate. Private calls, by contrast, target a single known user and fail if that user is unavailable.

Before first-to-answer calls, an originator facing this situation had limited options: initiate a full group call and accept that many recipients would be disturbed, or attempt sequential private calls and accept the delay. The first-to-answer mechanism, introduced with the initial MCPTT specifications in Release 14, provides a single-request, availability-driven alternative. It lets the network do the work of finding the first willing responder.

The mandatory manual answer is central to the concept's purpose. Automatic answer, while efficient in some MCPTT contexts, would defeat the intent of a first-to-answer call by establishing the call without any human decision. By prohibiting automatic answer, 3GPP ensures that the call is only created when a recipient has actively chosen to engage, which is important both for user consent and for operational discipline in mission-critical environments.