Since Best Buy does not sell T-Mobile devices and only their prepaid plans/SIMs you shouldn't need to unlock it from any carrier unless you activated a SIM from T-Mobile on it while you were here. You would need to make sure that you qualify here: Unlock your mobile wireless device
Unlock eligibility for Pay in Advance (Prepaid) phones, tablets, and mobile Internet devices
- The device on the requesting line has been active for more than one year on the T-Mobile network.
- If the device has been active for less than one year on the requesting line, the Pay in Advance account associated with the device must have had more than $100 in refills since device first use date. $100 in total refills is required for each device that is requested to be unlocked and does not include the initial activation payment.