Calls from AT&T customers will not route to our phone system -- all other carriers work

Should have been a migration.  But whatever it was isn't complete.  Call 611 

That was the first thing we did -- and was completely useless.  Every time we call in, they transfer us to Assurion, who in turn, transfers us to a number that just hangs up on us.

On rereading, I'm reversing some info... 

If you ported a physical number to a voice over IP number, you have to contact your new service provider because they are responsible for porting the number.  Them bouncing it back on AT&T is horse pucky.

   If it's now considered a virtual number, not a 'real' number, it may be flagged  by AT&T system which is why calls don't go through.

    But it's far more likely that the port was not completed by your v o i p provider. Please note that such ports can take 7 to 10 days. It's not like when you walk into a store and port a number between two physical carriers which happens within half an hour.

    Your new provider should be able to tell you when the port is complete, not blame your previous service provider went ports are entirely on the new provider.  

I think you're missing some of what I said.  The number in question was an AT&T cell number and the port definitely completed, because we were able to make and receive calls on that line immediately from everyone EXCEPT AT&T callers.  All of this took place almost two months ago, so there has been more than enough time for it to take full effect.  We receive calls on the number all day every day from non-AT&T customers.  There is zero question that this is a routing issue within AT&T's network.  We actually ported three numbers in total that day and the other two had no problems at all.

After reviewing the logs related to the porting of the number in question, AT&T support has acknowledged that there is definitely a routing issue, but they are unsure why and are unable to do anything with it because the number in question is no longer in their network.  After consulting with both AT&T and RingCentral (not the original provider we ported the number to), we are going to port the number back to an AT&T cell phone, verify call routing and then port it back to RingCentral.  They are both confident this should correct the issue.

According to Ring Central, It can take 15 to 30 days to complete the port on their end. Has it been anywhere near 30 days? 

The fact that it's working for some service providers and not AT&T tells me the port has not been completed, or not done correctly.

If you had bothered to actually take the time to read my post, you would have seen where RingCentral was NOT the original carrier we ported the number in question to.  This issue existed before we moved it to RingCentral.  According to AT&T, it sounded like the other carrier did not properly provision the number in some way when they received it.  We were having other issues with that carrier, so when we ported it to RingCentral, we had hoped it would force some kind of refresh, but it did not and RingCentral support says there's nothing more they can do to address it, as it only affects AT&T customers.

Also, when I port a number, it usually takes the receiving carrier a few days to confirm approval on the port request and then it is scheduled.  I have ported literally thousands of numbers over the course of the last 30 years and in not one single case has it taken more than 30 minutes for a port to actually take place -- sometimes hundreds of numbers at a time.  As an actual RingCentral customer and admin on my company account, I can tell you the port of our 12-15 numbers took 4 days for approval, we only had 5-10 minutes of downtime during the actual port and outside the issues with the number in question, we haven't had a single report of any customer not being able to call us as usual.

Did you ever get resolution? If so, are you able to share the root cause / resolution? We have a customer experiencing the same issue and we're struggling to get AT&T to resolve.