A communication system including a switching system which comprises a switching unit and a control circuit. The control circuit comprises a transmitting-side signalling handler procedure (ASH) for managing the connection and for exchanging messages with other procedures, and a receiving-side signalling handler procedure (BSH) for managing the connection and for exchanging messages with other procedures. For providing optional external services, the switching unit is coupled to a service control system. Furthermore, the control circuit comprises an additional procedure (ZUSA) for exclusively exchanging messages with the transmitting-side and the receiving-side signalling handler procedures (ASH, BSH). For specific messages, the additional procedure (ZUSA) is provided for exchanging messages by a service program (SERV) of the service control system and, dependent upon the messages produced by the service program (SERV), provided for sending messages to the transmitting-side and/or receiving-side signalling handler procedures (ASH, BSH).