My wife and mother in law dropping calls every time they talk and MMS won't send when WiFi is turned on

Hi - 

My wife and mother in law both upgraded to the Galaxy S20 in March 2020 2 days after the phone came out. Previously they were using Galaxy S8s. They had no issue with the S8s and now with the S20s, they drop calls to each other every single time they call each other since March. They can't send MMS pictures to each other if WiFi is turned on, even though they have Advanced Messaging turned on. Just to reiterate, these are two AT&T specific Galaxy S20s failing at the most basic of tasks, on the same network, on the same account. They can call other people no issue. I have no issues with an unlocked Google Pixel 4 XL either. Please help as none of this makes sense.