A communication system based on a communication protocol such SIP includes a first communication terminal Bob and a second communication terminal Bill, wherein the second communication terminal Bill has a unit, when the first communication terminal Bob receives an incoming message from a source communication terminal Alice, transmitting a substitute response control message to the first communication terminal Bob, the first communication terminal Bob has an incoming message transferring unit, when receiving the substitute response control message, transferring the incoming message given from the source terminal Alice to the second communication terminal Bill and has a response message transmitting unit transmitting to the source communication terminal Alice a response message containing contact information for specifying the second communication terminal Bill, and a call is established between the source communication terminal Alice and the second communication terminal Bill.