A message, sent in either direction during the setup or 
active phase of the call(ex.inbetween ACM and ANM), 
indicating that an event, which is of significance, and 
should be relayed to the originating or terminating access, 
has occurred.

Consider any call fowarding 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.