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

https://fun-wiki.win/index.php/Can_Carrier_Billing_Be_Used_for_Ringtones_Still%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