I'm a university student and recently I'm developing a Flash SIP softphone for web to SIP calls with the help of this sample program www.voip-sip-sdk.com/p_79-flash-sip-softphone-voip.html
provided by Ozeki VoIP SIP SDK. Using their manual I can easily build a Flash softphone application and a webphone server as well.
If you are interested in softphone/webphone developing, you can visit this site as they provide sample programs for many other VoIP solutions too. ;)