At beginning of this paper, it will introduce the SIP protocol major used on the VoIP service at present time. Introduce the development and the application of SIP. And then explain the operation of the SIP protocol by an example. Explain how a SIP client establish the communication with another one by the SIP proxy. After understanding the operation of the SIP, explain why the SIP client behind NAT will meet the fail and introduce the solutions that have developed to conquer the fail. Briefly describe each of these solutions and compare all of them. Next is to describe how the solution of statteful SIP proxy used to avoid the fail of NAT changes the behavior between the SIP clients from end-to-end to master-to-slave. The change of behavior between the SIP clients may affect the quality of the communication in some conditions. To improve this, this paper brings out an idea and implements it. Finally, test the quality of the communication, catch the packet carrying the SIP messages and packet of RTP, and watch the content of SIP messages and the routing of the RTP packet.