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

https://tiny-wiki.win/index.php/How_Did_Carrier_Billing_Start_with_Ringtones_and_Games%3F

Carrier billing lets you buy apps, games, or digital content and have the cost added to your monthly phone bill or taken from your prepaid credit. This means you don’t need a credit card or separate payment app