Carrier billing lets you buy apps, games, or subscriptions and charge the cost...

https://wiki-dale.win/index.php/Is_Pay_by_Phone_Billing_the_Same_as_a_Mobile_Wallet%3F

Carrier billing lets you buy apps, games, or subscriptions and charge the cost directly to your monthly phone bill or subtract it from your pay-as-you-go credit. It’s a simple way to pay without needing a credit card or separate account