Carrier billing lets you buy things like apps or music and charge the cost to...

https://bizzmarkblog.com/carrier-billing-for-subscriptions-is-it-easy-to-cancel/

Carrier billing lets you buy things like apps or music and charge the cost to your phone account. Instead of using a credit card, the amount is added to your monthly bill or taken from your prepaid credit if you have a pay-as-you-go plan