Carrier billing lets you buy apps, games, or services and charge the cost...

https://brookssnewinsight.scriblorax.com/posts/can-carrier-billing-still-be-used-for-ringtones

Carrier billing lets you buy apps, games, or services and charge the cost directly to your phone bill. If you’re on a monthly plan, the purchase will show up in your next bill. For pay-as-you-go users, the amount is taken from your available credit