Carrier billing lets you buy digital goods or services and charge the cost...

https://wiki-saloon.win/index.php/Does_Carrier_Billing_Work_if_I_Switch_to_a_New_Phone%3F

Carrier billing lets you buy digital goods or services and charge the cost directly to your phone bill. Whether you have a monthly plan or pay as you go, purchases either add to your regular bill or are taken from your prepaid credit