If you've been patiently waiting for carrier billing to come to your cell carrier, today might be your lucky day. Customers of Starhub in Singapore and A1 in Austria should now be able to charge app and content purchases from the the Play Store directly to their cellular accounts. A1 is the first carrier in Austria to support the feature, and Starhub is the second in Singapore.
Carrier billing is often a popular option in Europe and Asia where credit and debit cards are less common. If a carrier supports this feature, customers can purchase apps and content from the Play Store and have the charges added to their regular service bill. Not only does this eliminate the need to keep a credit card on file, but some services will also allow account owners to set spending limits to prevent children from racking up excessive purchases.
To make a purchase on your carrier account, tap the button to buy something from the Play Store, then tap on the price and "Payment Options" (or your localized variant). At the bottom of the list is an option to Enable billing with your carrier. Happy shopping!
Source: Google Play Store support