they rang for just 4 rings and then hung up. weird

Same as all the above.Called two days in a row now, and hang up when phone answered. So much for having an unlisted number.

Caller: Smart health australia

Had a few phone calls never left a message so phoned back and got a voicemail saying I have reached smart health Australia

Same here. They have called three times now. Rings for a short time. Doesn't leave a message.
I think I'll just block the number.

Same as for the other anonymous person's report. 4 rings and hung up.