The TPG-NBN VoIP will not work using a 3rd party modem/router. The modem we provide is standard modem and we’ve optimised the setup to complement our specifically configured modem settings. We provide the modem for all our NBN plans as they are sold as an Internet & Home Phone Bundle. As the Home Phone component of your bundle is encrypted for security, you will need to use our modem if you are to make and receive phone calls.
Please see my original response located here.