It doesn’t give you what POTS used to give you - several days of battery backup that kept the phone system working in the event of power failure. I remember the huge racks of lead-acid batteries in the old exchange buildings

The batteries were part of the trick, for sure. All of the telco gear ran on -48v, and was connected in parallel with the batteries.

In this way, grid power could come and go rapidly and the switches wouldn't even notice. There there was no handoff, or switchover time. The chemical batteries just changed from being held at a steady SoC to being discharged, which is something they can do without complaint or interruption. KISS.

After that, at least for the AT&T-built central office down the road from where I grew up, the generators would engage and keep things going indefinitely (or until fuel ceased to flow).