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

https://online-wiki.win/index.php/Can_Carrier_Billing_Still_Be_Used_for_Ringtones%3F

Carrier billing lets you pay for apps, games, or services by charging the cost directly to your phone bill. If you have a monthly plan, the charge appears on your next bill. For pay-as-you-go users, the amount is taken from your available credit