I would like to share my positive experiences with you. I am developing a WPF softphone in .Net and I have found a good sample program for .Net VoIP developers. With the help of Ozeki VoIP SIP SDK sample program I can easily make and receive calls, send and receive DTMF signals and navigate in IVR system.
If you are interesting in softphone developing here is the link for this website http://www.voip-sip-...phone-voip.html and you can start to build and configure your softphone.
And one more thing, it has a very good support!