SIP is exactly how 4G & 5G calls work already. It is perfectly technically feasible. Whether or not the 3GPP standard covers routers with backup data connections or not is a different question. But the router could, and should, be designed to recognise the number you are dialling is an emergency call and still route it via another carrier’s network. I was also responding to the other person saying it was a competition issue, but other carrier’s are mandated by law in Australia to take those calls.

Like, this was a solved problem literally decades ago. You could plug a phone into the jack and call 000 even if there was no service to the address. It is bloody shameful that this could have occurred in this day and age. There are plenty of rural places without any cell service where people rely on landlines in emergencies. The ACCC should come down hard on them, doubly so if it’s found out that anyone died as a result of the outage.