VOIP in C#

Question

Hi to all,

I need help to implement VOIP in c#2.0.
I searched VOIP SDKs and i need some more concepts to implement. At present VOIP SIP SDK(www.voipsipsdk.com)
I need Conference call and also want to monitor the current running call in live from another system (like call center conversation monitoring by Lead/Admin).

Thanks
Vishnu Prasad