The message which conveys this information is the DISC, which also carries a
cause code.
A DISC may occur because one of the users terminates the call (by hanging up
the phone, for instance). The cause code will be 'normal call clearing'. A
disconnection also may happen before the call has been completed, for instance
with cause code 'destination busy', or for some other reason.
This diagram shows the interaction at the end of a successful call.