You don't add a phone number to google voice to use the Obi. You set your google voice to forward calls to your Gmail's Google Talk, and then configure the Obi to answer those calls.
http://www.obihai.com/tutorials-tips.htmlYou must have a gmail account.
Your log in for gmail and google voice must be the same
You must have chat set up in gmail.
You must set gmail to receive incoming calls (install the voice and video plug in)
You must place one successful call to a normal phone number from within gmail
You must select Google Talk as a forwarding phone in your google voice.
Configure the Obi for your gmail account from the Obitalk dashboard.