A wired phone is only as good as what it is wired to...in my case it's
a cable modem so it won't do me any good if there is no power. I do
have one, and it has helped me determine if my cordless phone set is
the problem or if it's the cable co's equipment - once it was mine,
and once it was theirs. So mine comes in handy when there *is* power,
but won't help if the power goes out.

I do have 12V power supplies and inverters, so I could get the modem
up and running, but I'm not sure how far back into the cable co's
system power would be needed for me to be able to use the phone. I
guess a local outage wouldn't be a problem, but if it was widespread,
I assume that whatever the modem talks to would be down also.

As far as I know, if you have telephone over cable and the power goes down,
you're screwed until the power is restored. The same goes for cell phones
(at least in this area) no power, no cell phone. A standard old fashioned
wired phone connected to the telephone company over telephone lines will
still operate normally during a power failure.