I'm using a sample program providing by Ozeki VoIP SIP SDK to build my WPF softphone. I can create effective Voip solution with it and it doesn't require professional programming skills.
Furthermore, it has a really good and helpful support team! ;)
So if you are interested in Voip technology, you can find this site here: http://www.voip-sip-sdk.com/p_116-c-...hone-voip.html