Carrier billing lets you buy apps, games, or subscriptions and charge them to...

https://telegra.ph/How-Does-Pay-by-Phone-Work-on-Pay-As-You-Go-09-08

Carrier billing lets you buy apps, games, or subscriptions and charge them to your phone bill. If you have a monthly plan, the cost appears on your next bill. For pay-as-you-go users, the amount is taken from your available credit