Hmm, that's the first time I've heard about that approach with the NBN, and yes I think you're wise to be caucious about it.
Monty, if like us, you've checked the time frame for NBN and it's still some time away I'd ignore this approach, if you want to check and see if indeed Telstra or NBN did want to call on you.
Take a look at the following link, you can check actual avilabilty again, and there's a hotline number you could call and see if indeed Telstra were sending people to talk about NBN instalation: https://www.telstra.com.au/broadband/nbn
