As posted in other threads Telstra would not be alone in mandating use of their modem for NBN VOIP service. No way JB would necessarily have known although that brings up a good point in these times that they should have a list and ask the question about who your service is through, but the Telstra shop? A fail.
This brings up another aspect that if one changes providers from a BYO friendly to a BYO hostile RSP it should be done eyes wide open. I wonder how many NBN sales reps ask if someone is using a BYO modem on their existing service, or if they just advise the new one will be delivered in 3-5 business days without explanation?