File a FCC complaint, they are required to provide the batter for the eMTAs. This isn't MagicJack that gets away with it being non carrier grade VOIP. The battery backup ability was the whole reason the FCC permitted Cable VOIP.
The plant is designed to have backup generators to enable the phone system to still get a signal. If Insight loses power at their office, odds are that the whole town is and you'll have bigger issues to deal with.