T-Mobile added Apple Pay as an option for customers to pay their bills with, available on its website and in the app.
T-Mobile Apple Pay
You can use Apple Pay to set up automatic payments. Additionally, if you have an Apple Card you’ll get 3% cash back for using Apple Pay. So far it seems limited to paying bills, and Apple Pay isn’t an option for buying smartphones and accessories from T-Mobile.
Further Reading
[Apple Card Guide: All of the 3% Cash Back Merchants (Update)]