Since about February 2015, US Cell phone carriers have been required to unlock all phones that have been "paid off".

https://arstechnica.com/information-...ck-your-phone/

US carriers will now unlock your phone, thanks to law change | ZDNet

However, there is a new policy of blocking/bricking stolen phones (or phones with unpaid contracts). So, unlocking phones from the carrier is easy... as long as you actually own the phone without contract, or are maintaining the pre-existing contract while traveling.