For how long, 30 minutes to tide you through a thunderstorm, or 4-5 days to carry you through a major problem like in Lancaster a couple of years ago?
I have Verizon FiOS, which is the most common FTTP service in the US. The ONT they give us has a battery backup which will keep the whole service running for a few minutes, to get you through a quick power dip; after that it switches to a mode that just keeps the POTS lines running. That mode is supposed to last for about 8 hours.
Biting the hand that feeds IT
