实现sip电话穿越防火墙的服务器端代码，利用媒体中转的的方式使防火墙外的sip电话可以呼叫防火墙内的sip电话 -Sip call through the firewall to achieve the server-side code, using the media of the way transit outside the firewall can call sip phone sip phone within a firewall