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

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

Carrier billing lets you buy apps, games, or subscriptions and charge the cost directly to your phone account. If you have a monthly plan, the purchase appears on your bill. If you use pay-as-you-go credit, the amount is taken from your balance