A mobile phone connected to a telephone network which uses a telecommunication line by way of a communication terminal comprising: a communication unit transmitting and receiving data wirelessly; a sound input unit and a sound output unit which sound data are inputted to and outputted from respectively a transmission interface unit connected to said communication terminal through a transmission medium and transmitting and receiving data; a user information memory unit which is a smart card integrated circuitry configured to store user information which is unique information for the user information memory unit and constituted by built-in or detachably; a memory unit storing a phone call program for utilizing a telephone service which is provided by said telephone network; a detection unit detecting that the mobile phone and said communication terminal are connected mutually; and a control unit transferring the user information stored in said user information memory unit and the phone call program stored in said memory unit to said communication terminal through said transmission medium respectively and also changing over a sound data output destination of said sound input unit and a sound data input source of said sound output unit from said communication unit to said transmission interface unit respectively in a case when it is detected by said detection unit that the mobile phone and said communication terminal are connected mutually, wherein, when said control unit transfers said user information stored in said user information memory unit to said communication terminal through said transmission medium upon receipt of an authentication request from the communication terminal, the mobile phone is adapted to perform an authentication process between the mobile phone and said communication terminal and to provide the result of the authentication process to the communication terminal, wherein upon a positive comparison between the authentication process within the communication terminal and the authentication result received from the mobile phone, the sound output unit and the sound input unit are configured to communicate sound data to and from the communication terminal. a communication unit transmitting and receiving data wirelessly; a sound input unit and a sound output unit which sound data are inputted to and outputted from respectively a transmission interface unit connected to said communication terminal through a transmission medium and transmitting and receiving data; a user information memory unit which is a smart card integrated circuitry configured to store user information which is unique information for the user information memory unit and constituted by built-in or detachably; a memory unit storing a phone call program for utilizing a telephone service which is provided by said telephone network; a detection unit detecting that the mobile phone and said communication terminal are connected mutually; and a control unit transferring the user information stored in said user information memory unit and the phone call program stored in said memory unit to said communication terminal through said transmission medium respectively and also changing over a sound data output destination of said sound input unit and a sound data input source of said sound output unit from said communication unit to said transmission interface unit respectively in a case when it is detected by said detection unit that the mobile phone and said communication terminal are connected mutually, wherein, when said control unit transfers said user information stored in said user information memory unit to said communication terminal through said transmission medium upon receipt of an authentication request from the communication terminal, the mobile phone is adapted to perform an authentication process between the mobile phone and said communication terminal and to provide the result of the authentication process to the communication terminal, wherein upon a positive comparison between the authentication process within the communication terminal and the authentication result received from the mobile phone, the sound output unit and the sound input unit are configured to communicate sound data to and from the communication terminal.