This Annex describes how TS 23.237 mechanisms for IMS service continuity can be used to provide service continuity between on-network MC service and UE-to-network relay MC service. For illustration, MCPTT AS is considered as the MC service. Only the procedure for service continuity from on-network MCPTT service to UE-to-network relay MCPTT service is described in Figure B.1-1. The procedure for service continuity in the opposite direction is identical.

As illustrated in Figure B.1-1:

• Initially UE-1 has a direct connection to the network (on-network MCPTT service). It is registered with the SIP core and is engaged in a SIP session with the MCPTT Application Server (solid lines SIP-1 and MCPTT-1 in Figure B.1-1).
• When UE-1 realises that it is losing connection to the network, or after the connection to the network has been lost, UE-1 discovers a UE-to-network relay (UE-R) and establishes a PC5 connection with UE-R. UE-1 registers with the SIP core over the target access leg and enters UE-to-network relay MCPTT service by transferring the media streams over the target leg (dashed lines SIP-1 and MCPTT-1 in Figure B.1-1).
• The SIP session is anchored at a Service Centralisation and Continuity Application Server (SCC AS) before and after the handover, as described in TS 23.237.

Depicted in Figure B.1-2 is the call flow for service continuity when the UE switches from on-network MCPTT service to UE-to-network MCPTT relay service.