Carrier billing lets you buy digital content or services by charging the cost...

https://jaredsmasterinsights.nexorafield.com/posts/what-is-the-gsma-open-gateway-for-carrier-billing

Carrier billing lets you buy digital content or services by charging the cost to your phone bill. This means your purchase shows up on your monthly statement or is taken from your prepaid credit if you use pay-as-you-go