Lodge your complaint herehttps://www.donotcall.gov.au/complaint1.cfm

turns out it was telstra, calling about unusually high data activity on my phone.

Get a call from this number and also 8664 3800 presume they are connected - no message.  No ability to phone, telling me all lines to the area are busy - try again later.

I got a call today and answered it, but nobody was there and it hung up after five seconds.
I have a silent unlisted mobile number, so it's either someone calling a block of numbers or possibly someone linked to real estate rentals/credit checks - the last couple of weeks I've been inspecting and applying for new rental properties.

This number called me twice today and once yesterday, I had the phone on silent as I had done night shift. No message was left. I didn't bother to call back as I read all of the above comments and figured it was something dodgy.

I get a call from this number and others. It hangs up when I answer and it is busy when I call back. It is so frustrating.