A vehicle-mounted telephone device and its seamless automatic switching system and method with a mobile phone. The system is composed of a vehicle-mounted telephone device, a user's mobile phone, a call transfer center, a mobile communication network, and an external telephone system. The device is provided with a main controller, which is connected with a keyboard, a wireless communication module, a microphone array and an automobile ignition / extinguishment sensing signal device, and is provided with a call transfer information storage module and a call transfer request instruction sending module. In the call transfer center, there is a database for storing the vehicle phone number, the user's mobile phone number and the user's communication information. The feature of the method of the present invention is to utilize the vehicle-mounted telephone device and system to carry out the sequence control of seamless automatic transfer: when the car is started, the user's mobile phone can be automatically transferred to the vehicle-mounted telephone device, and when the engine is turned off, the call transfer is automatically canceled; When using the car phone to make a call, the other party's mobile phone will display the user's mobile phone number, so that when the user leaves the car, there is no missed call.