You're welcome @Zlex.
You are right, if you'll use a third party modem/router, then the VoIP service will not work.
To process a new NBN modem/router order, customers need to contact our Customer service team or through us (Community moderators) to help you place it.
Currently, we are providing either TP-Link VX420-G2v or VX420-G2h.
If you'll decide to get one with us, please send me a PM for us to check the price of it.
BasilDV
