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

https://online-wiki.win/index.php/Can_Carrier_Billing_Still_Be_Used_for_Ringtones%3F

Carrier billing lets you buy apps, games, or subscriptions and charge the cost directly to your monthly phone bill or subtract it from your pay-as-you-go credit. It’s a simple way to pay without needing a credit card or separate account