The feature is now supported by the carrier Three in each of the countries, in addition to SmarTone in Hong Kong and Telenor in Sweden.
The payment method enables 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.
Mobile phone billing is already available to customers of select carriers in Austria, Belgium, France, Germany, Japan, Italy, Norway, Russia, Singapore, Switzerland, Taiwan, Turkey, Saudi Arabia, and the United Arab Emirates.
Apple's support document explains how to set up mobile phone billing in the iTunes Store on both iPhone and iPad and Mac and PC.