Carrier billing lets you pay for apps, games, or services using your phone...

https://wiki-room.win/index.php/How_Can_I_Keep_Carrier_Billing_From_Messing_Up_My_Budget%3F

Carrier billing lets you pay for apps, games, or services using your phone account. Instead of entering card details, the cost is added to your monthly phone bill or taken from your prepaid credit