it depends on what you need and your budget. For top tier brand and model its better to get trade in @ US. Dont think any trade in program here, Its way cheaper with it.
If you only need a secondary phone just in case if main one die, non top model here is quite reasonable, my new secondary phone is about USD 200+.
sorry i have some info mixed up, i think i purchased a temp sim card with T-Mobil before i leave the US, since i remember GV required a real # (not virtual phone #) for the confirmation code when porting. Thats what i remember since its quite sometime ago.
i completely cancelled my US T-mobil plan about 1 month in advance and port the # to Google voice, since i wanted to test it and make sure everything works before i actually leave. i was only using the home Wifi for 1 month (but since i work at home so its ok). i purchase a Thai physical sim card at the AIS store here the day i arrived - of course with help of a local Thai. you can also get a temp esim in advance and activate it the day of boarding plane and it will auto connect once you arrive. and my plan is to get a temp esim if i need to visit the US.
have been using Google voice for 1 yr since i moved.
If you still have bank account or other US base services, better to keep the US #, and add a local Thai # on the same phone.
If travel back to US just buy a temp sim card for US only say 2 weeks just for internet and as long as u have internet service ur google voice will work.