AT&T's IMEI whitelist misidentifying your phone model and refusing to allow for VoLTE provisioning (more likely).
By the way, this reason quoted right here is why AT&T is such a frustrating mobile carrier now. Whitelists/Blacklists are imperfect at best, and can contain errors with whitelists being far more aggressively prohibitive than blacklists and prone to accidental exclusion. AT&T is using a pretty bog standard IMS/VoLTE provisioning, but they're using IMEI
whitelists to dictate who can actually connect to the voice network based on specific phone models, whether the phone can actually support VoLTE calling or not.
The reality of this decision means that even if a Verizon or T-Mobile branded and unlocked or global handset has support for AT&T's major LTE bands and VoLTE support, it may not actually work on AT&T. You'll note, the AT&T approved device list is mostly US flagship phones, US-based "global" handsets, or AT&T exclusive models. Intentionally or not, it effectively behaves as vendor lock-in that kills handset portability between US carriers, and blocks perfectly fine SIM unlocked phones that can and should work on their network from working.
Surely they need to have some suitable bands for foreigners roaming on US networks with their phones?
Like I said, there's a reason why most modern actual global/international handsets are just including LTE Band 13 support now. Verizon doesn't give a crap what phone you bring to their network anymore so long as the phone is SIM unlocked, not on a blacklist, and has support for their near globally exclusive C block Band 13 and IMS provisioning support to activate the SIM.
To kill handset interoperability in an era of what should be near universal handset interoperability here in the US:
-AT&T has restricted VoLTE calling support to an IMEI whitelist;
-T-Mobile has restricted VoLTE calling support through a non-standard implementation that requires software signatures;
-Verizon has restricted VoLTE calling support through the requirement of the phone supporting LTE Band 13, a band exclusive to Verizon in the US.
Of this, Verizon is the easiest to work around for international travel now, and most global flagships are accommodating for it. It also makes Verizon the most third party ROM support friendly, too. The 2020's are gonna go down as the "not the Onion" decade for all the events and twists it's held.
There's supposed to be roaming friendly and interoperability agreements between carriers here for bands 2/4/5 IIRC, but mobile networks are gonna mobile network.