I called back number (02) 6566 9366 spoke to a guy who said I've reached a private number.

Called claiming to be from Telstra and saying my computer was infected. After asking which office he was from, he said Miller St, Sydney which is actually a Telstra shop and not a major office (and a totaly different number). I called Telstra Miller St and was told there's no-one there of that name and that they would not cold call customers about such matters. I think that even though the number displayed, this is most-likely a scam.