you can use tmobile phone in nepal with US rates. Only caveat is, you have to stay in wifi and enable wifi-calling on your phone set. Out of wifi, you will be charged roaming charge by tmobile.
I ended up paying almost $50 in roaming even though I did not receive or call any number while out of wifi. It was kinda like data usage roaming, they charged me. I did not know that before hand. You should immediately disable your phone network while going out of wifi to stop getting charged of data roaming.
Besides that, while in wifi, I was able to call and receive any number in/from USA without a penny charge.