A main unit of a key telephone system accommodates a plurality of terminal units of the key telephone system that use a LAN within company as extension lines and exchange calls with outside lines. The main unit of the key telephone system has a function as an IP server for the terminal units and stores and manages a database of information in a storing device. A terminal unit of the system accesses an IP address of the main unit of the key telephone system, and logs in to the main unit with predetermined first ID and password. The main unit of the key telephone system authenticates the first ID and password and registers the terminal unit of the key telephone system as an IP extension terminal unit, and allows the KTS terminal unit to voice communicate with another terminal unit of the key telephone system in accordance with the VoIP.