I switched to Aussie Broadband a few months back but I have 3 left over optus routers/modems (replacements and changes in service means they gave us new ones from time to time). What should I do with them? No one has ever asked for them back and I don't have an active optus broadband anymore.
However you decide to get rid of them, make sure you do factory resets on them before they leave your possession.
Those routers can have sensitive info.
For example The new 5366TN keeps phone call logs, and the old 3864 can reveal your Optus VoIP details (i.e SIP credentials) through Admin access (which is not too hard to get).
And these are just what I can think of quickly of the top of my head.
Oh, for anyone that is still with Optus when they dump their old Optus routers, don't connect them to the internet through your Optus connection once its been factory reset because it will most likely re-download your SIP credentials back onto the router the first chance it gets.