Carrier billing lets you buy digital goods or services and charge the cost...

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

Carrier billing lets you buy digital goods or services and charge the cost directly to your phone bill. If you have a monthly plan, the purchase appears on your next statement. For pay-as-you-go users, the amount is deducted from your available credit