BASIC ISUP CALL FLOW

• 
  
  Initial Address Message (IAM) — First message sent to inform the partner switch (here MSC2) that a call has to be established on the CIC contained in the message. Contains the called and calling number, type of service (speech or data) and many more optional parameters.

• 
  
  Subsequent Address Message (SAM) — In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.

• 
  
  Address Complete Message (ACM) — Message returned from the terminating switch (here MSC2) when the subscriber is reached and the phone starts ringing.

• 
  
  Answer Message (ANM) — Sent when the subscriber picks up the phone. Normally charging starts at this moment.

• 
  
  Release (REL) — Sent to clear the call when a subscriber goes on hook.

• 
  
  Release complete (RLC) — Acknowledgement of the release – the timeslot is idle afterwards and can be used again. This is also sent (without a preceding Release message) if the terminating switch determines that the call cannot be completed. The terminating switch also sends a Cause Value to explain the reason for the failure, e.g., "User busy".