Sure, you can use any phone you want. Just transfer the SIM card from one phone to the other, They both use a nano size SIM, so there is no need to purchase a new SIM card.
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And to add on what e2k have shared, just make sure that your unlock phone has the necessary bands to work with T-Mobile. Non-T-Mobile phones will work on T-Mobile's 3G / 4G if they support Band II (1900) and/or Band IV (1700/2100).
^ Just here to confirm what these fine gentleman have to say.
An unlocked iPhone 6s will be a good choice.