Carrier billing lets you pay for apps, games, or services by charging the cost...

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

Carrier billing lets you pay for apps, games, or services by charging the cost directly to your phone bill. Whether you have a monthly plan or use pay-as-you-go credit, the purchase amount shows up on your next bill or reduces your available balance