Automated message.
Called around 3:15pm 14/7/2021, Sydney time.
Claim they are NBN and will disconnect the line within 24h.
I didn't wait to hear the rest of the message. Obviously would get to personal details somewhere along the way.
Just as a general note, all retail telephone/internet services are handled by your provider ( be it optus, telstra, aussiebb etc ), who in turn deals with nbn for any upstream issues. They will inform you of any issues and not nbn directly.