A local terminal device receives a local mail address of a local user, and converts the local mail address into a local domain name. The local terminal device transmits the local domain name and a local Internet protocol (IP) address to a dynamic domain name serve (DDNS) server to register with the DDNS server. The local terminal device receives a remote mail address of a remote user, and converts the remote mail address into a remote domain name. The local terminal device queries the DDNS server for a remote IP address corresponding to the remote domain name. The local terminal device exchanges VoIP signaling with a remote terminal device according to the remote IP address.