The present invention provides an improved SIP communication protocol. An NAT (Network Address Translator) traversal method is added before the SIP communication protocol, i.e. a client to client (C2C) module function is added to improve the function of SIP communication protocol, so as to solve the problem that the RTP (Real-time Transport Protocol) packets cannot traverse NAT firewall to achieve C2C communication after SIP (Session Initiation Protocol) is ended in VoIP. The major content of the present invention is to conduct a plurality of detection before SIP communication protocol, so as to predict the allocation rules of the port number by the C2C module, and open the RTP channel for C2C.