Carrier billing lets you buy digital goods or services and charge them directly...

https://wiki-zine.win/index.php/How_Do_I_Know_If_My_Mobile_Operator_Supports_Carrier_Billing%3F

Carrier billing lets you buy digital goods or services and charge them directly to your phone account. Instead of using a credit card, the cost appears on your monthly phone bill or is taken from your prepaid balance