I called back number (02) 9539 6899 reached IVR for a company called Open Agent.

Yes is open agent
Online form requires phone no

Very annoying!! I looked at their site and noted that I was not thinking of selling for at least 12 months and I get called at least 3 times a day when I don't answer. I think they call on a mobile number as well.

Answered and no-one on end of the line, then it just hung up.

Called 50 times in 2 days. I eventually unblocked them (my phone still notifies me if a blocked number has tried to ring through), and waited by my phone for them to call so I could tell them to take my number off their call list.

they call twice a day, machine speak goodbye when you call back

Caller: Open agent

They consistently call do not leave messages on message service, call up to 3 times a day including weekends.
Then hassle you if you are selling your home.

2 X Calls in a day, both times no one spoke