PURPOSE: To obtain the ISDN digital telephone system in which the talking charge in the case of extension talking is not required and B-channel blocking state is avoided.

CONSTITUTION: Information representing an extension talking request is set to a user user information element in any telephone set and a setup message with the user user information element added thereto is sent to a connector 2, then the connector 2 does not return the received setup message to the ISDN and the setup message using the content of an incoming address/incoming subaddress information element as an extension number is generated. Then the message is returned to the ISDN telephone set to call the message and both caller and called telephone sets are set to a same B-channel. As a result, the extension talking free of charge not through the ISDN is implemented by a reply of the called telephone set and in this case, since other B-channel is idle, the external talking and the extension talking are attained by other telephone set. Furthermore, in the case of the extension talking, the side tone of the relevant telephone set is adjusted to a proper value.