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).