I am using a OnePlus 6, still working perfectly fine, supports 4G and VoLTE ("HD Voice")[+]. AT&T kicked it off their network because it is not on their whitelist (The OnePlus 6T is). They sent me a branded LG Prime 2 as a replacement of a "equivalent" phone. It is actually a budget phone, not supporting 5G, and not even on AT&T's approved devices list (it seems they added it now, but last year it wasn't).

Forum users reported that customer service can whitelist an IMEI. They did not for me. So I switched to the T-Mobile network. From my international SIM now I get a message from their roaming partner AT&T saying that voice calls will not work.

T-Mobile is sunsetting 3G as well in July 2022, they sent me a text saying that they "think" that my phone is compatible. The decision of AT&T to not support my phone is not technical.

[+] It has 2 SIM card slots, something that seems to be impossible to get in the US. I think only one of the 2 slots fully supports LTE.