Is there a way that I can use this functionality with any desktop client especially something like empathy ? http://www.google.com/chat/voice/
I am referring the phone call function not just video chat which I know how to enable and use both in pidgin and empathy.
There was once an Adobe AIR-based desktop client for Google Voice available, but it is discontinued. There are instructions here for how to add it to Empathy, but it requires a gizmo5 account, which has since been acquired and discontinued by Google. In short, I don't think it is currently possible, with Empathy or any other client.