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.