22.0933GPPCompletion of Calls to Busy Subscriber (CCBS)Release 17Service description, Stage 1TS
The processing of CCBS requests shall not be affected by the mobility of subscriber A or destination B if either or both of the subscribers is a mobile subscriber.
Annex A (informative):
Cross Phase compatibility
Users of existing MSs (Phase 2 and possibly Phase 1) are not prevented from using the CCBS supplementary service.
Existing Phase 1/Phase 2 functional signalling mechanisms and USSD shall be used to offer the service to these MSs. Networks wishing to offer this service to existing MSs are therefore required to support both functional (Phase 1/Phase 2) and stimulus signalling (USSD).