Did you mean “not going over VoLTE?”
I wonder if the 3G calling was a case of implementing roaming over T-mobile… See: https://www.fcc.gov/wireless/bureau-divisions/competition-infrastructure-policy-division/roaming-mobile-wireless (The paragraph that begins “In 2011.”
Of course, maybe AT&T really hasn’t completed shutting down 3G. It sucks to be in the dark about all this stuff.
Unsurprising, given their whitelist/IMEI check.