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

https://highstylife.com/what-does-pay-by-phone-billing-mean/

Carrier billing lets you buy apps, games, or services and charge the cost directly to your phone bill. If you have a monthly plan, the purchase appears on your next bill. On pay-as-you-go plans, the amount is deducted from your available credit