Carrier billing lets you buy apps, games, or services and charge them directly...

https://blast-wiki.win/index.php/Carrier_Billing_for_Subscriptions:_Is_It_Easy_to_Cancel%3F

Carrier billing lets you buy apps, games, or services and charge them directly to your phone bill. If you have a monthly plan, the cost appears on your next bill. For pay-as-you-go users, the amount is taken from your available credit