Carrier billing lets you buy apps, games, or subscriptions and pay for them...

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

Carrier billing lets you buy apps, games, or subscriptions and pay for them through your phone account. Instead of using a credit card, the cost is added to your monthly phone bill or taken from your prepaid credit if you have a pay-as-you-go plan