ACM and ANM

Address Complete Message and Answer Message

An Address Complete Message (ACM) is sent in the "backward" direction to indicate that the remote end of a trunk circuit has been reserved.
The originating switch responds to an ACM message by connecting the calling party's line to the trunk to complete the voice circuit from the calling party to the called party. The terminating switch sends a ringing tone to the calling party's line. When the called party answers, the destination switch terminates power ringing of the called line, removes audible ringing tone from the calling line and sends an Answer Message (ANM) to the originating switch. The originating switch initiates billing after verifying that the calling party's line is connected to the reserved trunk.