It would have come from a scammer who can disguise the actual phone number they are calling from. It's called "spoofing" and is far too commonly occurring, in my humble opinion.
I just wish Telstra, or any of the other phone providers, would or could trace where these calls come from and bill them for each call they make. If nothing else, it would then actually cost them something and maybe encourage them to stop their criminal activity.

Anyone know who's number this is? The person said they didn't call me when I called back.