Quarantine from what I understand is where the number goes for several months before being released back to the public. If it was your number originally purchased through Optus then I can't see why you couldn't get it back. If you have a new phone plan with a new number they wont swap your number over if that's what you want to do. Best to speak with support on the live chat to see what can be organised.
https://www.optus.com.au/notices/service-chat?
