24.2063GPPStage 3TSVoice call continuity between Circuit Switched (CS) and IP Multimedia Subsystem (IMS)
The VCC application can be configured with any of various options for obtaining information from the IM CN subsystem specified in 3GPP TS 24.229 , 3GPP TS 29.328  and 3GPP TS 29.329 , for example:
a) receipt of REGISTER request which causes a third-party REGISTER request to be sent to the VCC application;
b) receipt of REGISTER request which causes a third-party REGISTER request to be sent to the VCC application. The VCC application then subscribes to the reg event package for that user to obtain information; or
c) receipt of REGISTER request which causes a third-party REGISTER request to be sent to the VCC application. The VCC application then uses the Sh interface to obtain information.
This document places no requirement on the use of all or any of these mechanisms.
6.2 VCC UE
There are no VCC specific requirements for registration of the VCC UE to the the IM CN subsystem.
NOTE 1: Depending on operator policy, registration to the IM CN subsystem can impact whether calls are delivered to the VCC UE using the IM CN subsystem or using the CS domain.
NOTE 2: The VCC UE performs registration in the IM CN subsystem independent of the UE’s state in the CS domain.
6.3 VCC application
The VCC application can obtain information from any received third-party REGISTER request, or any received reg event package, or the Sh interface, that it needs to implement the domain selection policy of the network operator.
If the third-party REGISTER request does not carry a P-Access-Network-Info header provided by the UE, and the VCC application requires this knowledge in for domain selection procedures, the mechanisms by which the VCC application determines the domain are outside the scope of this version of the specification, but could be dependent on configurable parameters in the DSF.