Patent ID: 7668305

Claim:
A communication system wherein a source communication terminal and a plurality of communication terminals, being connected to an IP (Internet Protocol) network, perform communications based on SIP (Session Initiation Protocol), the plurality of communication terminals including a first communication terminal and a second communication terminal, the second communication terminal comprising a unit to transmit a substitute response control message to the first communication terminal on the basis of a predetermined trigger when the first communication terminal receives an incoming message from the source communication terminal, the first communication terminal comprising an incoming message transfer unit to transfer to the second communication terminal the incoming message given from the source communication terminal when receiving the substitute response control message from the second communication terminal, and a response message transmitting unit to transmit to the source communication terminal a response message containing contact information for specifying the second communication terminal, wherein a call is established between the source communication terminal receiving the response message containing the contact information and the second communication terminal to which the incoming message has been transferred, the second communication terminal further comprising a unit to transmit a message for canceling the substitute response control message to the first communication terminal when receiving a transfer of another incoming message from another communication terminal than the first communication terminal, wherein the response message from the first communication terminal to the source communication terminal is not transmitted and the call between the source communication terminal and the second communication terminal is not established when receiving the transfer of the another incoming message from the another communication terminal to the second communication terminal.