99.999% is the standard for landline telephones, which I think is a pretty decent analogy here. Unless the customer has some crummy VoIP solution and doesn't trust their phones anymore :P.
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.
