Apple has enabled carrier billing in Belgium and Norway, expanding upon the feature's existing availability in Germany, Japan, Russia, Switzerland, Taiwan, Saudi Arabia, and the United Arab Emirates. The feature is limited to select carriers in each country — share if yours is supported in the comments.
The payment method enables customers to pay for iTunes content, App Store apps, iBooks, and Apple Music subscriptions without needing a credit or debit card, or even a bank account. Instead, purchases are added to a customer's mobile phone bill and paid off at the end of the month.
Apple has a support document explaining how to set up carrier billing on iPhone, iPad, or iPod touch and Mac or PC.