[gumby]

I know what you meant, but I feel compelled to be pedantic and point out they aren't really "self-powered", except the ones with a crank on the side, and even then that was just to initiate a call. The power comes from the switch it is connected to (though TBF to your turn of phrase they are "self powered" in the sense that the signal lines and power lines are the same).

As far as the old Bell System went: if you look, the old CO buildings have big parking lots around them (sometimes on all sides, and fenced off; other times actual parking lots on one or two sides. Those used to have huge diesel tanks in them (dunno if they still do this). The basements of those buildings we full of -48V batteries (dealing with H2 outgassing was an engineering issue). IIRC they were supposed to have a week's capacity, though that seems implausible to me.

Our current system is more capable but much less resilient.