Abstract: Relay devices T are installed in opposition to each other across an FW to implement an FW traversal communication between communication addresses such as IP addresses. Each relay unit 11, 21, . . . forwards and mediates a UDP communication performed with a communication address local to the FW beyond the FW by transmitting and receiving the DUP communication in a TCP simulated communication with a counter part relay device across the FW. Each call control unit 12, 22, . . . forwards and mediates a call control message for a call in accordance with the IP telephone (such as SIP) between a communication terminal device C11, C31, C32 or the like local to the FW, or a predetermined call control server (for example, an SIP proxy server or the like), and a counterpart relay device installed in opposition across the firewall. The TCP simulated communication enables the implementation of the UDP communication beyond NAT irrespective of the type and setting of the firewall and NAT.