Carrier billing lets you pay for apps, games, or digital content by adding the...

https://wiki-planet.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 digital content by adding the cost to your monthly phone bill or subtracting it from your prepaid balance. It’s a simple way to make purchases without needing a credit card or separate payment app