thanks for the reply. RBT is ring back tone. The REL message send the release cause as Unanswered call as per your comments. But is there any message in between REL and ACM whcihh tells the A side that the call is being routed to ann. and it should end the RBT.
Also can you please tell me what is the exact purpose and diff uses of call progressing message ( CPG ) in c-7 and in an E-1 connectivity how the status of trunks is maintained in CCS-7 n/w.