How the scam operates
One of the latest scams that is occurring is a missed phone call scam. Australians are receiving missed calls from international or 19/1900 numbers. The incoming call just rings once and then suddenly stops. Unknown to the missed call recipient, the call comes from a premium phone number. If this call is returned the scammer receives money, while the person returning the call will incur a high charge.
How to prevent this scam
The best way to prevent this scam is to not return missed calls to any country code that you are unfamiliar with or to a 19/1900 Australian phone number. If you get a call repeatedly from the same phone number, you can block the number on the iPhone. You can also block incoming calls from the same phone number on Android phones.
More information on this scam
Optus has some handy additional information regarding the missed call scam