说明：  osip协议栈高层API EXOSIP的API说明文档 该文档详细说明了如何使用EXOSIP库的函数来实现注册,呼叫,应答,挂机等电话流程. 资料相当难得,而且是最新的EXOSIP2的版本
(osip protocol stack API EXOSIP high-level API documentation of the document in detail how to use the library function EXOSIP to achieve registration, call answer, hang up, etc. call process. information very rare, and is the latest version of EXOSIP2)