To have a working phoneline, you will need to have the Telstra modem operating as the coding for the VOIP connection is hardcoded into the firmware and Tesltra is not releasing the details so that you could use it with another modem.
If you need the phoneline operational, then you could connect the Nighthawk to the Gateway via the WAN port on the Nighthawk, then run your wifi network from the Nighthawk.
If you don't need the phoneline operational, you could use the Nighthawk, but make sure it's firmware is updated to at least V1.0.0.32_1.0.1 before connecting (plug it in as above first and update it - or it could trip the circuit).