I think you are misunderstanding how Google Voice works.
Google Voice is an inbound call forwarding service, not a telephone company for OBi products. Assuming you have an inbound Google Voice number, and someone calls that number, Google Voice then forwards that inbound call to all of the forwarding destinations you have set up. These destinations can include up to six 48-state, 10-digit phone numbers, and any Google Hangouts clients that you are signed-into with the same Google/Gmail ID, with "ring on inbound call" turned on, and to "Google Chat". OBi devices act as Google Chat clients, so they are simply one of many possible forwarding destinations. You're not calling your "OBi's number".
I've been using 10-digit dialing for decades, and my Google Voice number, and all it's forwarding destinations, work fine. When you place a call from your OBi using Google Voice, always dial the full 10 digits.
If it isn't working, start by logging into your OBiTALK web portal, click on your OBi device, and check the status of the SPx that you set up with Google Voice. It should say "Connected".