Yeah, I did think about this more and realised it was NBN-based. But that’s kind of worse, at least on the NBNs part. Their systems would have noticed pretty quickly that they weren’t getting any traffic from Optus anymore. There’s no technical barrier that would stop them from being able to provide a failover endpoint to route outgoing emergency calls. Instead though, they spent how many millions on the bullshit backup batteries in fibre installs on the pretence that would help with emergency calls. All those batteries that die after 2 years, then the modem helpfully screeching regularly to let you know. Even if they spent all that money for a backup that only gets used on days like today, it would still have been more worthwhile.
Many of the consumer routers supplied by the ISPs now have failover to a 4g/5g connection, but it’s always the same network of course, so the failover is useless unless there’s some actual work done by the providers in allowing emergency failover to their competitors.
You’re completely talking out of your arse here though. That’s not how GSM networks or devices work at all. Optus mobile customers were still able to make 000 calls through competing operators. That is an international standard for GSM, and has been since the days of 2G. You can travel to ~80% of the world with your aussie mobile, dial 000 (yes, even in the US. yes, 911 works on an aussie mobile in Australia too) and if there’s a local operators signal, it will go through. You don’t even need to have a SIM card. There are provisions for data-only devices, but those routers can do calls over the 4G backup. If the device supports phone calls at all, it needs to support emergency calls through all available carriers to be compliant.