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

https://andresssmartcolumns.talesignal.com/posts/does-carrier-billing-work-if-i-switch-to-a-new-phone

Carrier billing lets you buy digital goods or services and charge the cost directly to your phone bill. If you have a monthly plan, the purchase shows up on your next bill. With pay-as-you-go, the amount is taken from your available credit balance