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

https://dibz.me/blog/why-do-carriers-cap-carrier-billing-daily-or-monthly-1245

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