afaik, neither tmobile nor at&t restrict based on device type, although NEITHER is configured correctly by default for APN/MMS, on any device (which makes sense since jolla doesnt test on or care about USA carriers). if your device supports the bandwidth, it will work on at&t and tmobile for 2G/3G/LTE…until next year when it will be data-only on LTE.
imo, VoLTE support is extremely unlikely on either tmobile or at&t, because even if jolla decides to give it to rostelecom, it will require someone porting it to usa carriers (its apparently not trivial to make VoLTE work for one telecom, even if its working for another)
in positive news, T-Mobile still hasnt shut down 3G, contrary to the rumor circulating last year
(EDIT: i know for a fact that at&t and tmobile both work on SFOS on Xperia X Compact as of now, for 3G and 4G, because i tested it on T-Mobile this morning, and on at&t a few weeks ago. highspeed data @8mbps+, calls/texts, MMS, all work. i am on Long Island, NY, USA)