@chaosmaou T-Mobile works well for me. I tried Ting first, but had issues. I didn't bother to troubleshoot knowing from others that T-Mobile worked. So, I just switched and happy with the results. Good luck!

The others phones that I know of: Xiaomi Redmi Note 9 Pro - sounds like VoLTE is working for people, but band compatibility may not be perfect. Xiaomi Poco X3 NFC - "" Various Volla - "" I've heard of people in USA using all three of these, but you really need to do your research and check with mobile providers. Brax 3 has a UT port that's coming along. Applee has been in touch with them. It seems too early to say though, which is too bad, because they have a December shipment coming along. Similarly Furiphone talked in the past of a UT port, but I haven't heard anything about it in a while. Both of those are explicitly made to work well in USA (among others). Also Fairphone 4 is supposed to have working VoLTE now? There was an ebay seller in USA who was selling Fairphone 4 new. NeverMSRP I believe. Ruben continues to work on VoLTE on the Oneplus Nord N10. Crossing fingers on that. Some people already have good results, but he's clear that it's still wonky so far.