It's most likely a scam according to fraudcreditcards.com:
https://fraudcreditcards.com/tmobile-web-order-800-672-5390-wa/
and our bank.
We had 2 charges for $11.99 and 2 credits in the same amount on our credit card. We called our bank, and they said it was probably fraudulent. They cancelled our card and reissued a new number and cards. Now, in less than 2 weeks, we have the *same charge on our new card.
Good luck!