T-Mobile's prepaid customers with Android phones get the Refill app for easy top-ups
You can view your monthly account details, schedule automatic top-ups, or make account payments. The app can also be used to send minutes and service to other prepaid users.
The app is valid for the Monthly 4G, Prepaid - Pay As You Go, and Prepaid - Pay by The Day schemes, and is a work in progress, as disclosed by its beta status
source: T-Mobile Refill (Play Store)