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

https://www.tumblr.com/magnificentlyultimatemist/827218616206475264/what-does-pay-by-phone-billing-mean

Carrier billing lets you buy apps, games, or services and have the cost added directly to your monthly phone bill or taken from your pay-as-you-go credit. It’s a simple way to pay without needing a credit card or separate account