Sorry about the late reply @Staca. Can confirm that you can reset by following these instructions:
When you access the VoiceMail service for the first time you will be prompted to change the default PIN number (The FAQ How do I setup Optus VoiceMail? explains this procedure). To change your PIN number after initial setup do the following:
Dial: 321 (free call)
Follow the Voice guided prompts for "Personal Options" (http://www.optus.com.au/business/mobile/solutions/mobile-services/voicemail)
If you're still needing a hand with this feel free to chat to us here → http://yesopt.us/chat2us.