That calling method is no longer supported by Google.
You'll have to add (or use an existing) 10-digit forwarding phone number to your list of forwarding phones in Google Voice Settings, and use that number to bridge the calls. If you have, or get, a SIP VoIP number (for example, a freebie from Callcentric), you can provision it on your OBi device and have Google Voice ring that number on your OBi-attached phone, as before.