Hi! I'm new to the forum. I just came back from my local store. All of the associates tried to help me process the JoD return, but they said their system would not allow them to do so and that I would have to contact customer care to obtain a return label since T-Mobile didn't ship one to me. Yes, I know they texted everyone to ONLY return it at the store, but I pre-ordered the 6s Plus over the phone and the associates think that could be the reason for the failed return. I also pre-ordered 2 additional phones at the store and I am able to process returns at the store for those. Besides the phone vs. in-store difference, what I also noticed is that a trade-in was also done as part of the in-store pre-order. This could be THE reason for the return problem. It's just a guess.
Anyway, I just finished an online chat session and I was sent a return label and a note was added to my account that I attempted to return at the store today but was unsuccessful. The rep indicated that if I was charged for both the old and the new lease I would see a credit on my bill once the phone is received.
TL;DR: I have to return the old phone by mail which is probably due to the phone rep not (properly?) processing a trade-in of the old phone as part of the pre-order.