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

https://mill-wiki.win/index.php/Why_Do_Payments_Fail_on_Prepaid_When_My_Balance_Is_Low%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