22.0933GPPCompletion of Calls to Busy Subscriber (CCBS)Release 17Service description, Stage 1TS
Activation of CCBS requests by existing MSs shall be performed using USSD. The standardised MMI is defined in TS22.030 . The network shall interpret the USSD string as an activation of a CCBS request.
Consequently, networks which do not support USSD will not be able to offer the service to existing MSs which do not support USSD will not be able to activate a CCBS request.