An adaptor serving for both a mobile phone unit and a private IP telephone unit is disclosed. A mobile phone unit is connected to the adaptor through a low power wireless system or a wired cable. A telephone unit controller manages resource data (data such as telephone directory, telephone history, and mail). A memory editor links a telephone directory of a mobile phone unit and a telephone directory of an IP-PBX in cooperation with a telephone directory controller of the IP-PBX. A VoIP extension line section provides the mobile phone unit with a function of an extension telephone unit in cooperation with the IP-PBX. A QoS controller is a circuit that performs control for minimizing loss of audio data due to communication congestion on a communication path (intranet/Internet). A charging circuit is a circuit that charges a battery of the mobile phone unit. The charging circuit contains a protecting circuit that protects the battery from being overcharged.