Do you have anything liked to 2FA? Bank Accounts, Email login?
That said its now a requirement for Telcos to send you a confirmation SMS to the phone before activating a port. If you don't reply the port fails. So it could still be that Optus have just stuffed up and disconnected your phone by mistake. They should be able to reactivate it easily enough. Contact them via face book or twitter and ask why it was disconnected. If it was a port, look at the above link.
Unfortunately Optus have no dedicated fraud number so you'll have to battle your way through the long waits to find an answer.