A method is performed at a computer server for enabling voice communication between two terminals through IM service. Upon receiving a message from a remote computer, the message including an IM identifier of a callee terminal user and a mobile device identifier of the terminal, the computer system updates an entry for the callee terminal user in a database. Upon receiving a request from a caller terminal user at a caller terminal to start voice communication with the callee terminal user, the computer server identifies the entry that includes the IM identifier of the callee terminal user and sends a voice communication request to a service control point (SCP) in a switched telephone network. The SCP then returns an IP address and a port number of the callee terminal and launches a call request against the callee terminal. The computer server establishes a voice communication channel between the two terminals.