Google discontinued support for that feature more than a year ago. There is no fix.
The workaround is to add a US 10-digit forwarding phone number to your list on this page:
https://www.google.com/voice#phonesThat number will appear when you use the extension, and Google Voice will call you on that number, and then it will bridge the call to your called party.
You can get a free inbound number from Callcentric to use for this purpose, and then provision it on your OBi as one of its Service Providers (SP1-SP4).