The other OBi devices do not need Google Voice numbers. Each OBi has a "burned-in", nine-digit OBi number, printed on the bottom. OBi devices can call other OBi devices for free, by dialing **9 followed by the desired party's OBi number.
You can also set up a list of trusted callers that can use the auto attendant feature on your OBi, by calling its number, and then their OBi number will be recognized and sent to your auto attendant. From there, they can make an outbound call using your Google Voice service.