I'm really interested in VoIP technology and therefore I am developing a Windows Form Softphone in Visual Studio with the help of Ozeki VoIP SIP SDK sample program. I have positive experiences with it because it is easy to use and provides ideas how to develop more functions into a softphone. You can check this program here www.voip-sip-sdk.com/p_136-windows-form-c-sip-softphone-voip.html
. You will find further information about configuration instruction and source code explanation.
Have a nice day!