Carrier billing lets you buy apps, music, or subscriptions and have the cost...

https://wiki-wire.win/index.php/Why_Do_I_Feel_Safer_Not_Typing_My_Card_Number_on_Random_Sites%3F

Carrier billing lets you buy apps, music, or subscriptions and have the cost added directly to your phone bill. If you use a pay-as-you-go plan, the purchase amount is taken from your available credit instead