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

https://sierra-wiki.win/index.php/What_Is_the_GSMA_Open_Gateway_for_Carrier_Billing%3F

Carrier billing lets you buy apps, games, or subscriptions and have the cost added to your monthly phone bill or taken from your prepaid credit. It’s a simple way to pay without entering card details, making small purchases easier on your phone.