A communication system for performing communication based on the Real-time Transport Protocol/RTP Control Protocol (RTP/RTCP) comprises a communication control device, a first NAT device and a second NAT device, a caller terminal device, and a callee terminal device. The caller terminal device transmits a first packet to the communication control device, receives a second packet that contains a second port number that is stored in the second NAT device, determines whether the second port number is one of an odd number and an even number; and transmits, in a case where it has been determined that the second port number is an odd number, an RTP packet to a port number that is computed by adding 2n−1 to the second port number, then transmits an RTCP packet to a port number that is computed by adding 2n to the second port number.