Carrier billing lets you pay for apps, games, or subscriptions by charging the...

https://wiki-neon.win/index.php/Carrier_Billing_Stopped_Working_After_I_Changed_My_Plan:_What_You_Need_to_Know

Carrier billing lets you pay for apps, games, or subscriptions by charging the amount directly to your phone bill. If you have a monthly plan, the cost appears on your next bill. For pay-as-you-go users, the purchase is deducted from your available credit