US Patent 9867090 Access transfer for a DRVCC mobile terminal

Dual Radio Access Transfer of an IMS in an alerting phase for the Packet Switched to Circuit Switched direction includes initiating an IMS media session towards UE and, during a session alerting phase, making a determination that a DRVCC transfer is required from a PS to CS, access, sending a SIP message from the UE to a SCC-AS, over the PS access, informing the SCC-AS of the DRVCC transfer, and responding to receipt of the SIP message at the SCC-AS by sending a SIP INVITE from the SCC-AS towards MSC. The SIP INVITE includes an identity of the UE as a called party identity and the STN as the calling party identity. The Transfer includes sending from the MSC to the UE a CS Setup message, whereupon the MSC is placed in a state in which it expects a CS Answer from the UE.