An electronic switching system having ISDN interfaces on the extension side stores in a table the attributes of a terminal under communication using the ISDN interface. When there is another incoming call to the ISDN interface, the system waits for a response to the incoming call by the terminals connected to the ISDN. When no response is received, the switching system creates a disconnection reason message on the basis of the result of checking the terminal attributes in the table against the attributes of the incoming call. This disconnection reason message is then communicated to the calling terminal.