Carrier billing lets you buy apps, games, or services and charge the cost...

https://wiki-net.win/index.php/Why_Do_Some_Checkouts_Say_%E2%80%9CPay_by_Mobile%E2%80%9D_Without_Explaining_It%3F

Carrier billing lets you buy apps, games, or services and charge the cost directly to your phone bill. If you have a monthly plan, the purchase appears on your next bill. For pay-as-you-go users, the amount is taken from your available credit