Carrier billing lets you buy apps, music, or services and charge them directly...

https://titusscoolinsight.capitaljays.com/posts/is-pay-by-phone-billing-the-same-as-a-mobile-wallet

Carrier billing lets you buy apps, music, or services and charge them directly to your phone bill. If you have a monthly plan, the purchase shows up on your next bill. For pay-as-you-go users, the cost is taken from your available credit