Carrier billing lets you buy apps, games, or subscriptions and pay for them...

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 pay for them through your phone account. Instead of using a credit card, the cost is added to your monthly phone bill or taken from your prepaid credit if you have a pay-as-you-go plan