Originally Posted By: Dignan
It looks like there's rumblings about people using SIP numbers to use Google Voice over IP. I'm not sure how easy it is, but it looks like it might be possible...

Its not an announced feature but you can call to a Google Voice number using SIP. It doesn't currently let you call out using Google Voice with generic SIP though. You have to use Gmail or have a Gizmo5 account from before if you want the ability to call out via VoIP only.

Read this post and linked posts for more information. It seems its not quite 100% as well.