Hi - I agree - its totally ridiculous that you need to keep the Telstra Gateway in the loop and I am not sure how that doesnt restrict the trade of other hardware vendors. I found this link which shows how you can configure it so the Telstra gateway is just a hang off of the modem/router and effectively only services the phone like a Cisco box would - http://www.ethertech.com.au/telstra-nbn-woes-with-voip-and-solution-for-3rd-party-routers/
Seems like overkill - I have a Netgear Nighthawk D7800 modem/router which is one of the best on the market and no way would I want to take a step back to the Telstra Gateway but until somone provides the needed VOIP setup data (which again I am not sure how they can't given they are selling us an NBN service not a modem) it will be very difficult to set anything up. You might find http://forums.whirlpool.net.au/archive/2425771 useful as well.
This information should be no more difficult to provide than router connection information.