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

https://victor-wiki.win/index.php/Why_Do_Mobile_Operators_Put_Spending_Limits_on_Carrier_Billing%3F

Carrier billing lets you buy digital goods or services and charge the cost directly to your phone bill. Whether you have a monthly plan or pay as you go, purchases either add to your regular bill or are taken from your prepaid credit