Thanks for raising this to us. 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.
If you wish to use your 3rd party modem/router, you may use it as Router Extension and maximize it's wireless feature. You may simply connect your own modem/router to LAN1 of our supplied modem and change it's mode to DHCP.
You may visit this existing article for additional reference.
Let us know how it will go and should you require further assistance.