Carrier billing lets you buy apps, games, or digital content and charge the...

https://mill-wiki.win/index.php/What_Is_Direct_Carrier_Billing_and_How_Is_It_Different%3F

Carrier billing lets you buy apps, games, or digital content and charge the cost to your monthly phone bill or take it from your pay-as-you-go credit. It’s a simple way to pay without using a credit card or entering payment details each time