Why You Should Buy an Unlocked or Verizon iPhone 8, iPhone X. As we saw with the iPhone 7, your best bet is to get a Verizon. (All Verizon phones are unlocked.).

How To Unlock A Budget Mobile Phone. For better clarification regarding iPhone X not being factory unlocked. Unlocked Iphone 5 Verizon Gsm. All Apple iPhone sold by Best Buy are NOT Sim-Free/Unlocked model.

What Best Buy sell is universal model. Those universal model will only work with U.S. Carrier such as AT&T, Verizon, Sprint, and yes T-Mobile. Best Buy do NOT sell carrier branded iPhone X or unlocked iPhone X. Reason why iPhone they sell are universal is because they are designed to work for one time activation to keep their inventory from overflowing. Once you activate with AT&T, it locks to AT&T. Same goes with Verizon, Sprint, and T-Mobile.

So if you buy iPhone X outright from Best Buy, yes you will be able to activate it with T-Mobile as it is supported U.S. Keep in mind, if you activate it with T-Mobile, it's going to be locked with T-Mobile. However, Best Buy is currently not selling iPhone X outright and requires activation only. Please be advised that while I am an employee of Best Buy, I am here on my own time just like yourself. I am here simply to offer insight and information from a regular employee point of view. My comments are my own and not representative of Best Buy Co., Inc.