I am also complaining in the strongest terms about my nbn fttc connection. While we were connected to asl we enjoyed years and years of faultless phone and internet service from Optus. But then the nbn arrived. Since it has been connected we have had nothing but trouble. I'm sorry but the Sagencom is a complete piece of rubbish (and they know it) and Optus should replace it with something better for their own sake (or give a choice to customers who want better WiFi coverage and no buffering etc, pay extra for a more powerful unit) We have drop outs repeatedly and a lot of buffering which we never had before and when you complain to service you will get the line "we will get back to you in 24-48 hrs" and guess what happens, nothing! I have a powerful Netgear asl/vasl modem which I cannot use anymore because it has no voip connection, so I have purchased a more powerful modem with voip connection included, and uptill now cannot use it because I need modem WiFi access point Voip account settings from Optus to connect it. With all the hassles we are having from Optus, it is getting to the stage of paying the exit fee just to have peace of mind and going with another isp. Other providers can sort out problems and allow you to connect byo modems without any problems, so why not Optus? In the end Optus will be the loser, as I and a lot of other customers (hopefully) have many many years ahead of them paying rent to isps for connection to the nbn.
You can connect your own "BYO Hardware", however you will be responsible for any problems/troubleshooting etc that may come up during the life it is connected. The downside, they do not release the SIP details to configure the VoIP side of things. Why? Because it costs more money to support more devices, VoIP can be setup over 1000 different ways, and unless your willing to pay a premium price for support, this is just how it has to be.
A few things to correct:
FTTC does not require a "modem". The NCD supplied by NBNCo is the modem, you simply plug an ethernet cord from the NCD to your preferred router.
You can continue to use the SAGEM for just the voice side. Simply plug your Sagem's WAN port into the LAN port on your preferred router, and forward port 5060 at the IP assigned to the Sagem device.
You just need to be aware, if you start having trouble, Optus will ask you to remove your router from the mix and test. If the problem continues, they can log a fault, if it does not continue, they can't do anything because their hardware and network connectivity is not faulty.