Setting up Payee information in Bill pay
With any bank, there are some payees that the bank doesn't have a digital payment agreement with. For those, what happens is they get the address and a contact number from you, and the bank sends out a paper check, via paper mail, to that payee for you.
I used this, for example, with my landlord a long time ago. It went to her in a plain white envelope with no distinctive markings - she tossed it the first time she got it because I didn't know what it would look like *grin* and couldn't warn her what to look for. I didn't even think about that possibility. You are not doing anything wrong; T-Mobile will just get a paper check, and you will need to allow a bit more time for the check to get there.
The good news on this is, you still have a free payment method, and you don't have to pay for the stamp. If you want it truly electronic and it won't work that way from the bank; you could go in each month to their (T-mobile's) website and have them do the equivalent of an EFT withdrawal which should also be free.