OK, this discussion has turned into a spitball-shooting contest.
When you set up Google Voice on an OBi, you are giving your OBi device permission to access the Chat service on your account.
OBi devices use the Google Chat service on your Google account to make calls over VoIP. Google Chat is not SIP.
If your Google account has a Google Voice incoming telephone number assigned to it, then Chat will use that number's caller ID for outbound calls. If the Google account does not have a Google Voice inbound number assigned to it, then it has no number to display, and thus, the called party will see "unknown caller" instead of a 11-digit CID.
That's what drgeoff meant. If your callers aren't seeing a caller ID, then either a) you don't have a Google Voice telephone number to send, or b) you do, but you set up your OBi while signed into the wrong Google account -- one other than that account that holds the number.