You'd have to dig deep in the forum to find it, but it's referenced here:
http://www.obitalk.com/forum/index.php?topic=7391.0I had originally ported "my" Google Voice number to an Anveo Personal Unlimited plan to get control of the number when it was unclear what the future for GV would be. I continued to use another GV number for "free" outbound calling.
About 15 months ago I switched to the Anveo for OBiTalk plan for no particular reason and, like you, discovered access to my SIP credentials was restricted. My VoIP needs are modest so it didn't matter to me.
Anveo doesn't refund prepaid subscriptions, but it's worth asking them if they will switch back to your previous plan and prorate what you've already paid for OBiTalk plan.
I may be wrong about this, but as I recall, you can only register one device to an Anveo account and sub-accounts are required, at an additional monthly cost, for each additional device.
I recently closed my Anveo account and ported my number to RingTo. For my needs, it's a better fit and the price is certainly right.
Good luck in getting your account straightened out.