Carrier billing lets you buy apps, games, or digital content and have the cost...

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

Carrier billing lets you buy apps, games, or digital content and have the cost added directly to your monthly phone bill or subtracted from your prepaid balance. It’s a simple way to pay without needing a credit card or separate account