1. Exhange A will send IAM meassage to Exch B
2. Exchange B subscriber will start ringing & Exch B will send ACM (Address Complete) meassage to B
3. As exch B get ACM it will switch Ring Back Tone to subscriber A
4. After Ring timeout Exch B will send REL (Release) to Exch A with cause B party not replying.
5. As Exch B get REL message it will disconnect RBT & play apprpriate announcement (Number U dialled is not replying)
There are many REL causes (e.g Busy,No answer,congestion,unalloted numbere..etc..etc),Details can be found in ITU-T specs.