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

https://juliet-wiki.win/index.php/My_Kid_Bought_Something_with_Carrier_Billing%E2%80%94What_Now%3F

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