I am getting the same error, have you had any success in this regard?
Just got it fixed today! When I upgraded to the X it shuts off Wi-Fi Calling!! Once that was turned by on NumberSync works! Only today's rep noticed that Wi-Fi Calling was off!
Previously, I was on the phone with AT&T Tech reps for hours, having me un-pair/re-pair AW, shutting off/on iPhone X and AW. One Tech about a week ago was very confident he could fix it but then gave up after hours and escalated my case and told me someone would get back to me. Of course, no one did. I called back a few days later to check up on the situation and another rep said they were still working on it, which I discovered today was a lie. I waited a few more days and talked to a rep today who said my case shouldn't have been escalated as it was an easy issue to resolve, and that a week ago the Techs simply reset things, so my case was resolved a week ago, so I guess the Rep that I talked to few days ago couldn't be bothered to look into things and simply told me to wait, telling me they needed more time.
When I upgraded to the X, NumberSync would work when I made call with AW, but after a while I found out I wasn't getting calls on AW when I was away from the X. Then a couple days later NumberSync completely didn't work. After a month after getting the X it is now working! It was so easy and hassle free when I set up AW3 with my old iPhone. I thought I had dodged all the problems I had read on here. Glad it's resolved now and hopefully they figure out a smoother set up procedure with the next AW. It's great being connect and not always needing to carry the X.