This number rang my mobile number and after a short pause a recorded message said (more or less) "This is Nicole from the NBN. We are going to disconnect your landline as the NBN is now in your area. Press 1 to speak with a technician".
I did nothing and they hung up after a total of 33 seconds.
I did nothing because my number is one of dozens of on-call numbers for my national organisation, so I am sceptical they really were from the NBN.