Description
Private Call Call-Back (PCCB) is a standardized supplementary service within the 3GPP Mission Critical Services (MCS) framework, specifically defined for Mission Critical Push-To-Talk (MCPTT). It operates within the IP Multimedia Subsystem (IMS) layer of the network. The service allows an MCPTT user (the originator) who attempts to establish a private call (a one-to-one call) with another user (the destination) but finds them busy, unreachable, or not answering, to invoke a PCCB request. The network then stores this request in a PCCB state.
Architecturally, the PCCB service logic resides within the MCPTT application server (AS), which is an IMS application server enhanced for critical communications. When a private call setup fails due to the destination's status, the originating MCPTT client can send a SIP INVITE request with a specific indication for PCCB, or a subsequent SIP request like MESSAGE. The MCPTT AS, acting as the PCCB service provider, acknowledges the request and monitors the availability status of the destination user. This monitoring leverages registration and presence information from the IMS core. Once the MCPTT AS detects that the destination user is registered and available for MCPTT service (e.g., not in another MCPTT session and has service coverage), it automatically initiates a new private call setup between the two parties, fulfilling the callback.
Key components involved include the MCPTT client on the User Equipment (UE), the MCPTT AS hosting the PCCB service logic, and the underlying IMS core providing registration, session control, and presence capabilities. The service works by maintaining a PCCB record containing the identities of the originator and destination, the timestamp of the request, and its state (e.g., pending, being processed). The MCPTT AS manages the lifecycle of this record, including expiration timers to prevent indefinite pending states. Its role is to enhance the reliability and success rate of critical one-to-one communications by automating retry logic, ensuring that important calls are not missed due to temporary unavailability.
Purpose & Motivation
PCCB was created to address a critical operational gap in push-to-talk communications for public safety and mission-critical workforce users. In high-stakes environments like emergency response, it is imperative that communication attempts succeed. Traditional cellular calls or basic PTT services simply return a busy signal or go to voicemail if the recipient is unavailable, which is unacceptable when urgent instructions or status updates need to be delivered. PCCB provides a guaranteed delivery mechanism for private call attempts.
The problem it solves is the inefficiency and potential failure of manual retry. Without PCCB, a user who encounters a busy colleague must repeatedly attempt to call, wasting time and attention during critical incidents. PCCB automates this process, offloading the retry task to the network. This was motivated by the requirements defined by professional mobile radio users and standard bodies like TCCA, which were integrated into 3GPP's MCPTT specifications starting in Release 13. It addresses the limitation of previous PTT systems, which often lacked intelligent, network-managed supplementary services.
Furthermore, PCCB increases the effectiveness of team coordination. By ensuring that a callback request is queued and executed automatically, it allows the originator to continue other tasks with confidence that the connection will be established. This aligns with the broader 3GPP mission to provide carrier-grade, feature-rich critical communications over LTE and 5G networks, replacing or complementing legacy systems like TETRA or P25 with more flexible and IP-based services.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (155 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-14, normative work from Rel-15.
In Release 15, the PCCB (Private Call Call-Back) function was newly introduced, enabling remotely initiated private call procedures for both client and server. This included enhancements for managing functional aliases between the serving and owning MCPTT servers, and introduced specific call control corrections for handling private answer modes and off-network private call types. Additionally, the release defined procedures for upgrading a standard private call to an MCVideo emergency private call and for supporting associated emergency alerts.
- Remotely initiated private call client procedures TS 24.379CR0356
- Remotely initiated private call server procedures TS 24.379CR0357
- Managing functional alias – MCPTT server serving the user TS 24.379CR0390
- Managing functional alias – MCPTT server owning the functional alias TS 24.379CR0391
- MCPTT Emergency location triggering criteria TS 24.379CR0469
- Adding audio media level section in SDP body for offnetwork MCVideo group and private call TS 24.281CR0072
+ 3 more changes
In Release 16, the PCCB function was enhanced with new capabilities for restricting incoming private communications and for using functional aliases in private calls, involving updates to both client and server procedures. Additionally, the release introduced mechanisms for the cancellation of a private call prior to setup and made corrections to the off-network private call control state machine. These updates provided greater control and flexibility for Mission Critical users initiating and managing private call-backs.
- Restricting incoming private communications - call control TS 24.379CR0481
- Using functional alias in private calls – client procedures TS 24.379CR0482
- Using functional alias in private calls – server procedures TS 24.379CR0483
- List of MCPTT group members who did not acknowledge the group call request TS 24.379CR0486
- Update service authorization procedures to support limiting the number of authorized clients per MCPTT user TS 24.379CR0552
- 24.379 Copy MCPTT ID Fix TS 24.379CR0465
+ 7 more changes
In Release 17, the PCCB function was enhanced for MCPTT and MCVideo services by introducing new call control features for private communications, specifically call transfer and call forwarding for MCPTT private calls. Furthermore, it defined comprehensive interconnect procedures for MCPTT private calls between networks, including the functional specification for an MCPTT Gateway Server. These additions expanded the flexibility and interoperability of private call handling beyond the capabilities of earlier releases.
- Call control - Restricting MCVideo private communications TS 24.281CR0105
- Interconnect - MCVideo Private call procedures TS 24.281CR0157
- FA as a target user for MCVideo private call TS 24.281CR0177
- Add altitude, timestamp to MCPTT location XML schema TS 24.379CR0625
- Call transfer for MCPTT private call, call control part TS 24.379CR0673
- Add accuracy to MCPTT location XML schema TS 24.379CR0698
+ 49 more changes
In Release 18, the PCCB function was enhanced to support private calls to a user that has migrated to a partner system, ensuring call continuity and session establishment across administrative domains. Furthermore, the release introduced procedures for location information requests from an MCPTT client, including requests using a functional alias, to better support mission-critical operations. These updates expanded the interoperability and situational awareness capabilities for private calls within the MCPTT service.
- MCPTT Chat group join to group regroup TS 24.379CR0855
- Addition of 5G MBS inter-RAT information in MCPTT signalling TS 24.379CR0873
- MC GW UE functions in MCPTT TS 24.379CR0901
- General adhoc group call procedures in single system - Protoc impl for MCPTT TS 24.379CR0902
- Add the description of 5MBS in MCPTT TS 24.379CR0899
- Support MCPTT over 5G ProSe TS 24.379CR0905
+ 55 more changes
In Release 19, enhancements for the PCCB (Private Call Call-Back) function were introduced through clarifications and corrections to the MCPTT private call transfer and forwarding procedures. These updates specifically refined the signaling and operational handling for transferring a private call between users. The improvements ensure more reliable call control and user plane separation during these transfer operations.
- Modify list of participants by changing the criteria during an ongoing MCPTT ad hoc group call TS 24.379CR1004
- MCPTT adhoc group call to migrated user TS 24.379CR1015
- Adhoc group call release by an authorized user (MCPTT) TS 24.379CR1022
- FRMCS_Ph5 Adding reason to leave a session in MCPTT TS 24.379CR1029
- Enhance handling of criteria for ad hoc group communication (MCPTT) TS 24.379CR0996
- MCPTT Prevent multiple participations in a group regroup TS 24.379CR1008
+ 11 more changes
Explore further
Broader topics and technologies where PCCB plays a role.
Defining Specifications
3GPP specifications that define or reference PCCB, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 24.281 vj40 | MCVideo Signalling Control Specification | Rel-19 |
| TS 24.379 vj50 | Mission Critical Push To Talk (MCPTT) call control | Rel-19 |
| TS 37.579 vi40 | Mission Critical services conformance testing | Rel-18 |