you can use your phone anywhere in the world. You might be missing the point. You can keep your old US cell phone and use it only when needed. Buy another phone here and put in a Thai SIM card. Done
turn off data and roaming and always use on wifi. Only when you HAVE to use it though. Otherwise keep it off. I’m fortunate in that my whole family and most my friends in the US all use iPhone. So I have my Thai sim and can FaceTime and text message to them for no cost.
I would never buy my phone here. I use iPhone and apply pay. Thais can’t use Apple Pay. If I buy a phone here, I can’t use Apple Pay. Thus, I buy in the US. But your phone there and when you get here, get a Thai SIM card for it. Keep your old phone with your US number in case you need it. Been working for me for years. I don’t notice the $60 dollars I spend to keep the US number.