It sounds like you were on a lease plan. The were recently shut down as a lot of people may have been inadvertently mislead into thinking they were a "better" plan than a standard buy your own phone plan (they weren't)
If you do nothing then Optus will assume you wish to keep the phone and add a purchase amount to your bill. This will be similar to the second hand price of the phone on the market now. Don't know what phone you have but could be $400-$1000 or so.
So at a guess around $600 perhaps. Optus should give you an exact payout figure on request. But before that you need to work out if you were on a LEASE plan. These plans were identical to BUY YOUR OWN PHONE plans except
1) You got a $10 monthly discount on your bill 2) You had to hand your phone back in great condition at the end (or buy it outright)