How can I make/receive Google Voice calls in Mountain Lion? I use Google Voice as my main line now adays. However, having to leave Google Chrome open and logged into Gmail is getting in my way...

I was wondering if anyone has a way to get these calls to go to a VOIP app that is installed on the computer. Does such software exist?
The telephone app in the app store is a great one to use, i'm using it in mountain lion and it works perfectly, was by far the easiest softphone I've ever set up. http://itunes.apple.com/nz/app/telephone/id406825478?mt=12