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

https://front-wiki.win/index.php/Is_Carrier_Billing_Good_for_People_Without_a_Credit_Card%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 shows up on your next bill. With pay-as-you-go, the amount is taken from your available credit balance