Carrier billing lets you buy digital products or services using your phone...

https://mighty-wiki.win/index.php/Carrier_Billing_vs_Debit_Card_for_Small_Purchases

Carrier billing lets you buy digital products or services using your phone account. Instead of entering credit card details, your purchase cost is simply added to your monthly phone bill or taken from your prepaid balance if you have a pay-as-you-go plan