I just got off phone with an Apple Genius, she said before powering the phone on remove the T-Mobile sim and replace it with the ATT sim. If the phone gets powered on with TMobile sim, it WILL be locked to TMobile. Anyone who is having phone sent to house will be fine just so long as they remove sim but if you are having ship to store they are required to power phone on which will lock it! Hope this helps and I hope this is true!!
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