Carrier billing lets you buy digital goods or services and charge the cost...

https://brookssnewinsight.scriblorax.com/posts/can-carrier-billing-still-be-used-for-ringtones

Carrier billing lets you buy digital goods or services and charge the cost directly to your phone bill. Whether you have a monthly plan or pay as you go, purchases either add to your regular bill or are taken from your prepaid credit