A call prefix is an important property of the call service. The call prefix defines the number rule of the call connection, and reflects the numbering and routing solution of a switch office. The call prefix determines the service type of a dial rule (a basic service or a supplementary service; an office internal service, a local call, or a toll call), and specifies the length of numbers. The call prefix can also be used to control call rights.
A private branch exchange (PBX) system verifies the validity of the numbers dialed by users based on call prefixes, and processes the numbers. Proper call prefixes are critical to a successful service configuration. Users' calls must match call prefixes so that calls can be connected. One or more call prefixes may be matched during a call (a basic service or a supplementary service).