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

https://yenkee-wiki.win/index.php/What_Is_the_Simplest_Explanation_of_Carrier_Billing_for_a_Friend%3F

Carrier billing lets you buy apps, games, or services and pay for them through your phone bill. Instead of using a credit card, the charge is added to your monthly bill or taken from your prepaid phone credit