> One old fashioned safety check was that in the old days your landline
> telephone system was sustained by independent power from the telephone
> company. Don't know if this is true.

It was definitely true. The exchange was battery-backed, the handset was (and still is) powered down the wires, and there was nothing but wiring between the two. It only failed if the wires got cut, or all the operators were on strike. In a disaster, at least that's a start.

I remember being on a tour of a small Strowger (mechanical) telephone exchange half a century ago. The power backup was 24 lead-acid cells in series, each cell being basically a five-foot cube; how long this would last, I forget, but it was certainly enough to paper over short power cuts. (Since each cell produced just over two volts, it also explains where the 50V standard for telecoms kit came from.)

How much of a modern exchange is battery-backed, I know not; and long-distance lines these days are usually digital. Those planning such things these days are far too used to mains electricity being reliable.