Calls that we make from our desk phones to AT&T mobile numbers DO go through. Calls made from AT&T mobile numbers TO any of our company phone numbers do NOT go through. AT&T Wireless customers get a busy signal or a "We're sorry but all circuits are busy" message. Sprint and Verizon callers do not seem to be affected, and are able to call us just fine.

I called RingCentral support, and I was told that maybe it is because all 50 of our phone numbers had somehow been flagged as spam. This doesn’t make any sense for a number of reasons:

If we are flagged as spam, why are we still able to call AT&T customers? Shouldn’t they have blocked us from calling their customers instead of the other way around?

How did all 50 of our phone numbers get reported for spam simultaneously? We have only publicized about 10 of these phone numbers. How would someone find the others?

The chat support in the RingCentral admin does not work. The virtual assistant said an agent would be arriving shortly but that was over an hour ago.
The phone support person put me on hold forever before coming up with the “flagged as spam” theory.
And the form to Submit a case ( https://support.ringcentral.com/new-case.html ) says I don’t have permission to create a ticket, and the “Next” button stops working on the second page after I’ve typed in my ticket description. So I guess the only method available to me for any support is a community forum.