What is the need of CPG message in ISUP protocol?

Question

It is becoming very difficult for me to recall it. Do anyone knows about this message flow? All answers are welcome.

Answer 1

CPG message that Call is in Progress.
Lets have a example considering a call forwarding scenario.
As soon as switch receives Setup, it checks that the Called_Party has forwarded option activated and so tries to forward the call. Meanwhile, Switch sends CPG message towards originator saying that Call is in Progress.