A fundamental concept for Intelligent Network (IN) control is the basic call state model (BCSM). When a call is processed by an exchange, the call goes through a number of pre-defined phases. The BCSM is a high-level finite state machine description of the Call Control Function (CCF) activities required to establish and maintain communication paths for users who have IN services. For circuit switched operation there will be an Originating Basic Call State Model (O-BCMS) and Terminating Basic Call State Model (T-BCSM).