In the process of porting my brother's number from Virgin to Optus, Optus has given that number to and it has been activated by, some random stranger. My brother is now no longer the account holder for his own mobile number and they can't give him the details of the person who is.
Not only is this a massive breach of my brother's privacy and raises serious questions about Optus account security, but after 3 days the best Optus can suggest is to speak to Virgin to reverse the port, despite being an error in the Optus system.
Which means whoever has activated my brother's mobile number will suddenly no longer be able to access it.
Although this is assuming Virgin can reverse the port because the account holders details are now different and you can only reverse a port by visiting a virgin mobile store, which no longer seem to exist.
Unfortunately Optus don't have any real process where you can contact someone who is authorised, interested and will take ownership of the issue.
FWIW I would suggest you jettison the number. Make sure any security accounts that are linked to the number get updated to whatever new mobile number Optus give you. SMS your contacts to update your number. You can even call your old number and let the new owner know of the stuff up an tell him to refer callers to your new number.
IME getting either Optus or Virgin to fix this will be a massive headache.