A mobile communication system provides a phone number maintaining service. In the system, a first mobile switching center (MSC) receives a call request signal with calling terminal information and called terminal information, detects an initial service provider of a called terminal from the called terminal information, and transmits the call request signal to a communication network formed by the detected initial service provider of the called terminal. A second MSC detects a changed service provider of the called terminal based on the called terminal information included in the call request signal transmitted from the first MSC, and transmits the call request signal to a communication network formed by the detected service provider. A third MSC transmits the call request signal received from the second MSC to the called terminal so that the called terminal can communicate with the calling terminal over a communication network formed by the changed service provider.