Absolute horrible phone call quality in house on multiple phones with WiFi-calling turned on.

Now, this might be an issue with AT&T, but I'm trying to understand how the WiFi-calling with iphones works to help troubleshoot better.

I have a client who can barely make phone calls in their house with their iphones (AT&T) along with their guests when they have them over. I've instructed them to turn on wifi-calling on their phones, and they still have the same issue. Calls get dropped, and their voices go to that robotic staticky sound. I can assure you they have an extremely strong wifi signal in their house, we have 12 access points installed and they have 200Mpbs internet speed.

Now, I've realized today that Facetime video and Facetime audio work absolutely perfectly. Facetime video is crystal clear video and the audio sounds great, and it works perfect when I make a strictly audio Facetime call. But when I call back normally, its the same issues over again.

This leads me to believe that this is some issue with AT&T, but if they have wifi-calling turned on, then shouldn't that negate any cell tower signal issues they might be having? Once again, this is happening to multiple iphones, they are all updated to the latest iOS, and have been hard rebooted.

Any information or ideas to get me further in the troubleshooting stage would be helpful. Thanks!