Carrier billing lets you buy apps, games, or digital content and charge the...

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 digital content and charge the cost to your monthly phone bill or take it from your pay-as-you-go credit. It’s a simple way to pay without using a credit card or entering payment details each time