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.
Hi, my experience has been that if the payment is being sent by check then something in the biller details is incorrect. I had a credit card (Chase) that they did this for and it turns out that I had put in the company address when the address needed was "Cardmember Service" address from the statement. Once I changed it to that, the payment went through electronically.