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

https://jaredsmasterinsights.nexorafield.com/posts/how-can-i-keep-carrier-billing-from-messing-up-my-budget

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