Partially a topical subject with the Pukekohe situation, but also a response to a posting elsewhere on the Internet.

I imagine the Pukekohe incident is even more galling where the failure of the 5 Nines PSTN occurred while broadband services and the telephony alternatives it supports continued to operate.

The point I guess is that less complex systems with intelligence at the edge can survive when the complex central switch has a conniption.

Don't get me wrong, in its time, where one system was all that could efficiently be supported, a system used by first responders and emergency services, 5 Nines was essential. If you couldn't effectively build a two-tier system, the one had to be as reliable as possible.

"If your reliable system depends on reliable human beings, it is not reliable" - if it can function in the presence of unreliable humans, it has a chance.

Centralised systems have a single point of failure and despite all the attempts to protect it, without redundancy, it will fail.

In addition to VoIP services, cellular, and how few Nines that service provides I shudder to guess, but as a back up when the "telco grade" service failed, I suspect it was very helpful.

It may be that at the technology stage of that system, centralisation is the only risk/cost/benefit option, but keep an eye out for the day when two independent 99.9 systems are cheaper (or even a little more expensive) than one 99.999 system, do the math, buy as many 9s as you need.