how to create voice chat in wpf

I'd start by looking at peer to peer WCF services:
http://msdn.microsoft.com/en-us/library/cc297274.aspx[^]

I think you should reconsider you architecture: it will have scalability problems as the traffic goes through the server. I better solution, depending upon what you need, might be

1. When client "logs in" the machine is resistered at the central server
2. When one client calls another it gets the information it needs to connect from the central server
3. The client creates a peer-to-peer connection with the called client. .

This way the central server is not involved with the conversation. The main drawback with this set up is that it is more complicated to develop.