Would it have been less "snarky" if I padded my correct answer with some
happy-faced help desk language, like: "Thank you for your post! I'm so glad you asked, and I am delighted to assist you! If I understand your question, then <repeats the user's question> and offers a scripted answer..."?
It's not a 7-digit vs. 10 or 11-digit issue. The 503 error code means the service is unavailable. It's unavailable because you're using an Apps domain account.
OBi devices use the old Google Chat/Talk (XMPP) protocol. Google is phasing out Chat support across its product line. It's unclear from Google's documentation whether or not Chat/Talk has been completely removed from Apps accounts at this point in time, but you can log into your Apps domain console to look for yourself. I don't have an Apps domain, so I can't test this myself. I will say that using GV with Apps domains is strongly discouraged. It is too easy to accidentally delete or otherwise modify the Apps account, resulting in losing the associated GV number permanently.
If it
is still available to you, then you can try enabling it. You will also need to set up Google Voice on that account, including adding Chat as a forwarding destination by making an outbound phone call from a web browser, using Chat. Again, it's not clear if this will continue to work on an Apps account, but if you're motivated, you can try it.
https://support.google.com/a/answer/60767?hl=enObihai substantially updated their device firmware last year to conform to Google's authentication requirement. Direct account sign-on (passing a user name and password from OBi device to Google) has been deprecated by Google and is now replaced by exchanging secure access tokens via OAUTH 2.0. So, downgrading to an old OBi firmware version will not solve your issue at all. The fact that you see the service as "connected" merely means that the OBi device was able to successfully access your Google account, not that you can use it for calling. Any posts made prior to last spring are no longer valid nor relevant.