If you’re on a prepaid or pay-as-you-go plan, your phone is probably unlocked; however, if you’re on a two-year contract or something similar, your phone is probably carrier-locked until you finish paying it off.
The most reliable way to determine whether or not your phone is unlocked is by calling your current carrier and asking them to verify your phone’s status. You will most likely need to provide your account password (if applicable) and your phone’s IMEI number. You can find this on most phones by dialing *#06# on your keypad.
Phones need to be unlocked prior to activating your Mint Mobile service.