Pure old copper-line phones did not need a working power outlet in your home - electricity was introduced to the system at facilities of the phone company (often miles away), and often used a dedicated electric grid.

With VoIP, you tend to have your own, plugged-in wall box. Which becomes useless the moment your electricity stops, e.g. in case of natural disasters.

Also, analog telephony just sounds better than digital one. No downsampling happening.

and if you want to dial 911 with a land line, you literally only need a speaker and resistor connected to the phone line. you can yell into it as a mic, and just tapping the line on and off dials a number.

Has anyone tried plugging all of their internet-related equipment into a UPS, including the wall box? Will that allow you to keep VoIP working in the event of a power outage, or is it a pointless exercise since the ISP's equipment on the pole will also be down?

I did this during our last power cut. The ISP was unaffected. Kept a few table lamps going too.

Analog telephones have terrible bandwidth. Just 300–3,300 Hz. This it technically called the voiceband, but it cuts off a good portion of female speech.

Also, all phone lines these days, wether POTS or VOIP are digitally trunked and switched.

Not to mention the exchange usually had tens or hundreds of lead acid batteries which could keep it running for a long time.

Analog telephony doesn’t get used anymore. As soon as the call hits a telco box it’s going digital

It's not really about cost. It has implications on cost, but…

Some spare parts have been out of production for decades. If one of those parts needs replacement, then keeping an exchange running may now need replacing much more than just the broken part, or the technicians may try using parts from other exchanges (the number of landlines is much lower than at peak, so there are spare spares in many exchanges, so to speak). That's an expensive way to maintain a network, obviously.

VoIP infrastructure is more fragile than copper assuming equal hardware age and comparable maintenance regimes, but I assume that the lack of spare parts changes that.

I think that for most households, mobile phones are increasingly the replacement for landlines. That requires mobile operators to keep at least a share of their cell towers online, which seems easier than keeping the physical line to each household online in the event of a disaster.