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

https://charlieeocd131.lucialpiazzale.com/why-do-mobile-operators-put-spending-limits-on-carrier-billing

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