Definitely a spammer. Likley an automated spam polling system to see if your number is active. If you answer then they will later spam you from another number.

Got a busy signal when i rang it and my PABX displayed a 'wrong number' message.

I called back and it also rang three times and cut off. Probably another damned telemarketer!!

This number rang me twice, when I called it back about a half hour later I got an Optus message saying it was disconnected?

I called back number (02) 9993 0002 rang 3 times then gets disconnected automatically.