Hello, upon receiving terminating invite SCCAS would check if the subscriber is ims registered or not. If its not ims registered it it would perform a user data request for csrn number And route the request to cscf where bgcf function route the call.
If the subscriber is ims registered, it would intiate diameter udr request reueating tads information to hss. Hss based on the subscription and location of subscriber would return two parameter which are ims over ps session true/false and subscriber location which could be ps or cs. If the location is ps the invite would be routed back to scscf which would route the call to terminating pcscf. Othetwise if the location is cs or ims over ps session is not supported the subscriber would be routed to cs using csrn routing same as unregistered case.
