Basic information about a VoIP SDK

I am a student and i have to prepare a writing about the basics of softphone development with the title: Which are the most important aspects of building a softphone? I also need to add a real VoIP SIP SDK to it. I found an SDK: Ozeki VoIP SIP SDK on the net: http://voip-sip-sdk.com/p_272-how-to-build-a-softphone-...., but there are so many information about it so I cannot select the most important ones. Would you be so kind to help me with giving the most important aspects of those things I should concentrate on?
Thanks.