Carrier billing lets you buy apps, games, or digital content without a credit...

https://lima-wiki.win/index.php/Carrier_Billing_vs_Credit_Card:_Which_is_Better_for_Security%3F

Carrier billing lets you buy apps, games, or digital content without a credit card. Instead, the cost is added to your monthly phone bill or taken from your prepaid balance if you use pay-as-you-go