Difficult to know for sure as nearly every foreigner I've run into uses a local sim, but it likely will be AIS or TrueMove (True or True-H or TrueMove H).
Other than for accepting an SMS or emergency call, you may be better off with a local SIM (50-100 baht [$1.6-$3.2]). It will usually come with some call credit + free internet for a couple of days, possibly a week or so. You can get extra monthly packages for 200+ baht or the "pro second" plan where you pay internet per mb (25 satang per MB) and voice (1.2 satang per second) for local calls (100 satang = 1 baht; 30 THB ≈ 1 USD); no monthly fees.
If you don't have a dual-sim phone, you would need an extra phone for this strategy if you wish to have both sims active at the same time.
edit: after evaluating T-mobile's roaming packages for Thailand, it would seem that you have the "T-Mobile ONE and Simple Choice North America" plan, then your Text and internet is covered (calls are $0.25/min).
But those with "other" plans must first sign up for "World Class Service". You get $15.00/MB (fifteen USD per Megabyte). $0.50/SMS (outbound) / domestic plan balance (inbound). Voice calls still $0.25/min.
The coverage area seems to be consistent with AIS/True: