The phone grid predated the electrical grid. There was no other choice for power.

Actually, there was one. Even earlier phones had their own power. A dry-cell battery in each phone, and every 6 months, the phone company would come around with a cart and replace everyone's battery. Central battery was found to be more convenient, since phone company employees didn't have to go around to everyone's site. Central offices could economize scale and have actual generators feeding rechargeable batteries.

It's a pretty decent chunk of power down a POTS cable too, as it was designed to ring multiple big chunky metal bells in the days of yore.

I was wiring in a phone extension for my grandma once as a boy and grabbed the live cable instead of the extension and stripped the wire with my teeth (as you do). I've been electrocuted a great number of times by the mains AC, but getting hit by that juicy DC was the best one yet. Jumped me 6ft across the room :D

Grid charging batteries, phone draining them as I understand. Of course there were switches all over the us so I can't make blanket claims but from what I hear that was normal.

The batteries and phone lines were one system at -48v with power supplies converting AC power to DC while grid / generator is up.

The batteries are floated at the line voltage nothing was really charging or discharging and there was no switchover.

This is similar to your cars 12v dc power system such the when the car is running the alternator is providing DC power and the batteries float doing nothing except buffering large fluctuations stabilizing voltage.