I just got a call from a mystery Sydney number. I missed it, but a message was left. I listened to it, and it was a recorded message saying that I'd won a $100 sports voucher, and to press 5 now. My first instinct was 'Free shit? Yipee!', so I pressed 5. All I got was the regular voicemail voice saying 'your message has been saved'. I've heard about this stuff. Should I be worried?