Carrier billing lets you buy apps, games, or services and have the cost added...

https://delta-wiki.win/index.php/How_Do_I_Check_for_Unexpected_Charges_on_My_Phone_Bill%3F

Carrier billing lets you buy apps, games, or services and have the cost added to your monthly phone bill or taken from your prepaid balance. It’s a simple way to pay without using a credit card or entering payment details every time