Be interesting to see if all the people having issues have WiFi calling enabled. I've been having issues, and I have WiFi calling active.
It would make sense too. Wasn't a huge deal when it was only a few people with iOS 9, but with the masses having access to WiFi calling and the fact that AT&T hasn't officially released it? Couple that with it makes sense why only AT&T people are having the issue. No other carrier is soft launching their WiFi calling along side iOS 9.
Tempted to disable WiFi calling and unprovision all the different SoC codes it adds to your line on your account when it activates to see if that is truly the problem. We would need to do it on a wider scale.
Yes, I might happen to be employed by the company in question. Yes, WiFi adds about 3-4 "SoC" codes to your lines rate plan that only we can see. No, it doesn't cost anything. Yes, they can be removed if needed.