As of this week, the feature is now supported by Movistar in Chile, and dtac and AIS in Thailand. Elsewhere, Vodafone and O2 has been added to the list of supporting carriers in Netherlands and Czech Republic, respectively.
The payment method allows Apple customers to pay for iTunes Store content, App Store apps, iBooks, and Apple Music subscriptions without needing a debit or credit 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.
The payment method is already offered by select carriers in Austria, Bahrain, Belgium, Bulgaria, Cambodia, Croatia, Denmark, Estonia, Finland, France, Germany, Japan, Hong Kong, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malaysia, Norway, Poland, Qatar, Romania, Russia, Singapore, Slovakia, South Africa, Spain, Sweden, Switzerland, Taiwan, Turkey, Saudi Arabia, the United Arab Emirates, and the United Kingdom.
Apple's support document explains how to set up mobile phone billing in the iTunes Store on both iPhone and iPad or Mac and PC.