A modem to process calling packets includes receiving a calling request packet from a software phone of a communication terminal, and determining if the calling request packet includes a special tag. If the IP phone is idle, the modem records a source IP address of the calling request packet, and modifies the source IP of the calling request packet to be an IP address of the IP phone, then the modem transmits the modified calling request packet to a server, and receives a calling reply packet from the server, then modifies a destination IP address of the calling reply packet to be the IP address of the communication terminal. The modem transmits the modified calling reply packet to the software phone to establish the call.