My phone automatically flagged the number as spam. There was a message left, though that might have been accidental. In the saved message, I could hear multiple people talking, perhaps in a call centre. Someone evidently didn't hang up right away. I am not going to call back and neither should anyone else.

Claimed to be from Telstra and said some bullcrap about my IP Address being public. Told him where to go.

Phone blocked/flagged caller as suspected spam/reported as telemarketer. They did not leave a message. Do not answer, do not call back. Just block and delete.