I believe it's what they design for at the central switch nodes. The buildings would have entire rooms if not floors dedicated to nothing but 48 volt wet-cell batteries.

Of course the "last mile" infrastructure is not so reliable. That said, in over 20 years I've had POTS service, I can't ever remember not having dial tone when I lifted the handset.

Not noticing downtime is not the same as not having it.

I'll sell you 100% uptime at a much better price if you promise to only check whether you're up a few times a day...