A few points on this (to the best of my understanding). Although note that this article is from 2016 so this isn’t a new question.
The original FTTP solution specced a UPS for the NTD (basically the box inside your house). That little box provided you with a standard “copper” port to plug your basic old phone into as well as an ethernet port for you to plug your router into. In theory the battery would allow your phone to keep on working during a blackout.
Is your phone cordless?
Because even if the NTD has power, you’re still screwed if you don’t have fixed handset plugged in.
How do you get your phone service?
Many providers provision a phone line not over the “phone port” of the NTD but through a plug in the back of the router. Effectively it’s a little VOIP system built into the router. This saves them having to pay NBN for a voice port as well as a data port.
The UPS on the NTD was only designed for the NTD itself, not other things (like the router that may be providing the phone port). Unless you have your own UPS you’re screwed here anyway.
NOTE: This is the same approach historically used for Cable connections from Optus, rather than pay to use the Telstra copper they run the phone out the back of their cable modems (I believe Telstra also do this in some places too).
Are you on FTTN?
This one I’m not 100% sure of. Can yous till use the “normal” copper for phones or does it use the same approach as above running the phone out of the modem?
In any case, the same limitations apply when it comes to cordless phones.
It’s worth noting that there are also batteries needed in the FTTN cabinets. These local nodes have batteries which are supposedly good for 6 - 8 hours of uptime (in short your local UPS will likely go flat first).
Personally, I’m on cable and do have a UPS, but it’s only good for maybe an hour… although to be fair that’s after shutting the server down after 20 minutes and while keeping the whole network up (APs and network switch). If I moved everything but the modem and phone onto it, it would run considerably longer… but then I don’t have a ‘land line’ phone plugged in anymore anyway
The vast majority of the time there isn’t any power outages that last very long so a small and cheap UPS would work just fine in providing piece of mind. To go seriously beyond that would start to get far far more expensive looking at something like the Tesla power wall and solar panels…
Plan B might be to replace the land line with a mobile phone (which may be cheaper anyway given most plans are now unlimited calling). Then add in some hardware to make her feel more comfortable if needed. Something with a decent dock for it to live on like the old cordless phone does should make it reasonably acceptable. The beauty of this is that it has it’s own built in UPS!
You can get cordless phones with Bluetooth that can connect to your mobile for a more “traditional” house phone look and feel. (although you then need to power it).
You can even buy cabled handsets like this to make it really retro
I’m sure it’s possible to all but hide the fact it’s a mobile if it makes her too uncomfortable.