ok, got an Xperia 10 III. VoLTE on T-Mobile USA works just fine.
i was able to get 3Mbps download (normal LTE speed in my area) while maintaining a voice call.
however, there are mobile network internet issues that are quite annoying. IPv6 issues seem to account for most of them, and workarounds make it tolerable so far.
ALSO; i tried an already activated T-Mobile 5G simcard, and it was able to make calls and send texts without T-Mobile blacklisting it. i didn’t verify VoLTE on the new simcard, but i have no reason to think it would not work just fine