If I am not mistaken you might want to wait for the phone to come to you and then send that phone back in the box with the label provided. Those trade-in offers are supposed to come with return labels. That label might also be what TMobile is expecting to receive so that you get trade in credit. I know that when you order a phone in a store that they tell you to wait. However they will usually take a trade-in at a corporate store if the phone is in stock and you're doing it on the spot. However with pre orders and backdated deliveries with almost absolute zero in stock availability for the iPhone x this would not be the case.
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