Carrier billing lets you buy apps, games, or services and charge them straight...

https://wiki-site.win/index.php/Why_Do_Payments_Fail_on_Prepaid_When_My_Balance_Is_Low%3F

Carrier billing lets you buy apps, games, or services and charge them straight to your phone bill. Whether you have a monthly plan or use pay-as-you-go credit, the cost is added to your next phone bill or deducted from your available balance