Seem be having issues of late with at least 3 voicemails a day coming through with no missed call evident and no message on the voicemail notification. The phone will notify of a voicemail via the standard text but there is no missed call and when you listen to the voicemail as it registers there is no message left. It's now becoming somewhat annoying on my Apple Iphone whilst my other phone which is Android doesn't have this occur. Anyone experienced the same?
Try putting your iPhone SIM into the Android device, if the problem reoccurs then it's your SIM that is faulty, get any Optus store to replace it.
However, if you do this swap and the problem clears up, then your iPhone is not connecting to the Optus network properly and is faulty. You'll need to start troubleshooting that, depending on the model, the steps vary a little bit. The first step would be to reset the network settings (this does wipe out saved WiFi network tho), SETTINGS > GENERAL > RESET > NETWORK SETTINGS (This does not remove any content or personalisation, just saved WiFi networks).