An apparatus to back up heterogeneous terminal user information in an IP terminal includes a terminal connection interface, a temporary synchronization storage unit, a synchronization controller, and a data converter. The terminal connection interface connects to a heterogeneous terminal. In the apparatus and its method, the temporary synchronization storage unit temporarily stores phone number information of the heterogeneous terminal. Upon connecting to the heterogeneous terminal, the synchronization controller sets synchronization with the heterogeneous terminal and stores phone number information of the heterogeneous terminal. The data converter performs data conversion between the stored phone number information of the heterogeneous terminal and stored phone number information of the IP terminal.