Hello Naveed Ahmed,
After reading through your question, I think this Codeplex project can help you: http://csharpsoftphonevoip.codeplex.com It's based on Ozeki VoIP SDK (www.voip-sip-sdk.com) that support all .NET languages including
C# as well.
Although your question has been posted more than a year ago, but I hope it can be useful for your readers as well! :-)
All the best