Google Voice is an inbound call forwarding system, not a free phone company. If you live in the 48 contiguous US states, and you have a valid/working US-based telephone number you can submit to confirm your eligibility, then you can get a free inbound Google Voice number from Google's pool of numbers. Alternatively, you can port in a US-based mobile number to Google Voice.
Once you have your own Google Voice inbound telephone number, it will be used as your caller ID on outbound calls.
If you make calls on an OBi while signed into a Google account that does not have an inbound Google Voice phone number, then your called party will see no caller ID (the CID field will instead say "unknown" or "private caller" or "restricted").
If you want to use some other VoIP service provider, then that's another way to get a number and use it for caller ID, or to spoof a number as your caller ID. Google Voice does not permit caller ID spoofing.
Here are my complete instructions for setting up Google Voice on an OBi:
http://www.obitalk.com/forum/index.php?topic=8560.msg56460#msg56460Please refer to the Google Voice Help Center to learn more about the service:
https://support.google.com/voice/#topic=1707989