If you wish to be familiar with SIP protocol implementation then I suggest you to first read more useful information about the background of the VoIP technology. I used some examples for my project as well. I have found this explanation earlier:
The Session Initiation Protocol (SIP) is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol. SIP helps in establishing communication sessions which involve various media types such as audio, video, or collaborative multimedia.
I mean, SIP protocol is very important to use when we would like to build an audio or video based application. (For example: Ozeki VoIP SIP SDK is appropriate to create SIP VoIP call services.)
The info derives from this webpage: voip-sip-sdk com/p_304-sip-protocol-implementation-voip html. I always use this website because this SDK has the best documentation which is plain and transparent to understand it.