Carrier billing lets you buy apps, games, or services by charging the cost...

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

Carrier billing lets you buy apps, games, or services by charging the cost directly to your phone bill. If you have a monthly plan, the purchase is added to your next bill. If you use pay-as-you-go credit, the amount is deducted from your balance