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.