Carrier billing lets you buy apps, games, or digital content using your mobile...

https://station-wiki.win/index.php/Why_Do_Carriers_Cap_Carrier_Billing_Daily_or_Monthly%3F

Carrier billing lets you buy apps, games, or digital content using your mobile phone. Instead of a credit card, the cost is added to your monthly phone bill or taken from your pay-as-you-go balance. It’s a simple way to pay without entering card details.