Carrier billing lets you buy apps, games, or services by charging the cost...

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

Carrier billing lets you buy apps, games, or services by charging the cost directly to your phone bill. If you have a monthly plan, the purchase is added to your next bill. If you use pay-as-you-go credit, the amount is deducted from your balance