Cannot make and receive phone calls AGAIN!!!! Phone whitelisted ( at&t brand)

One phone that I used for years got blocked by at&t could not make phone calls, purchased an at&t phone calls worked for 3 weeks ... Can't make phone calls again... How to resolve it? I am about done with them. The phone is on the whitelist, I tried everything, apn settings, resetting networks, calling them, new sim cards,...G8X ThinQ (LM-G850UM) is my device. It is an at&t device. It clearly is on their approved list of devices-funny thing is that i think what triggered it , was me putting "mint sim" wirless card for an hour into that phone, as soon as I put their card back into the phone , I could no longer make or receive phone calls. I looked up on youtube and someone said att&t needs to provision the phone on their network.. but their technicians do not know what they are doing. I think it is intentional that they do not train their technical well.Long story short I have ben havving issues with their network for like 3 months, no phone call service ( the only reason I did not take care of it right away is because I was in Europe for a while and was using a local network that worked perefctly fine with my phone that would not work with at&t)