Overseas scammer using a spoofed (fake) national number, "Telstra" Technical Department". You have a virus on your computer. They call regularly, they must enjoy the chat with our answering machine because they keep calling.

Woman called from this number.
She told me that my computer was allegedly interfering with the internet.
She wanted me to switch my computer on so that she could send me an "application", which would enable her to "scan my computer to find the cause of the interference".
I led her on a little, but eventually told her that she was a scammer, whereupon she hung up.

Nobody at the other end. Maybe they were just finding out which of their auto dial numbers answered?