Hi,
I have recently read about a C# open source VoIP softphone. SIP (Session Initiation Protocol) which is the essential pillar of VoIP technology. The task of SIP protocol is to reconcile the attributes of telephone sets and soft phones, and then it establishes a telephone line between two or more parties who want to communicate. It is very useful to have an SDK. Several SDKs were tested and the best one was the one by Ozeki.
It is a stable and reliable SDK. I can strongly recommend it and you can read about it at
http://www.voip-sip-sdk.com//p_116-h...-sdk-voip.html