Multiple calls, no message left.

Unanswered call - no message left. I phoned back but recorded message advised the number has been disconnected.

Called and didn't leave message

Australia n institute of research. something to do with property ownership

This person called my private home phone number. How did they obtain this number ? After answering they hung up. I tried returning the call and had a Optus notification stating that this number is disconnected. How can someone call on a number thst is supposed to be disconnected ?