Asterisk 1.8 Released With Support For Google Voice
Thinkcloud writes with a note that long-standing open-source VoiP software Asterisk has just been updated, and it's packed with more than 200 enhancements, security updates, and new features — including calendar integration and support for Google Voice and Google Talk. Asterisk's fully-featured PBX includes call waiting, hold and transfer, caller ID, and other useful tools so it's a great option for small businesses that need to watch costs."
As others have pointed out already you can surely do this with Asterisk, but I think in your case the question is whether your 3G network operator allows SIP traffic on their unlimited data plans.
OS Reviews: Free and Open Source Software
That word... I do not think it means what you think it means.