For security reasons I highly recommend not using the number you plan to drop when setting up Google Voice. Use your cell phone number to set it up. Make sure you get a new Google Voice number. It will let you choose the number. There is also a "lite" version of Google voice that does not have incoming calling available. You don't want the "lite" version. After you set it up make sure your cell phone is not checked as a forwarding phone. Make sure Google Chat is set as a forwarding phone.
Those options are under settings in Google Voice.
Now click edit under your cell phone
Click "show advanced settings"
Set "Voicemail Access:" to " No (requires pressing star during greetings -- default for non-mobile phones)"
Without logging out go to
http://www.google.com/chat/voice/ You must make one outgoing call from Gmail. You may need to install the plugin to make the call.
Finally add the Google Voice account to your OBi.