I read about backup for VoIP on a leading Magazine today. One of the backups it suggest is that to keep the good old copper wires, PSTN, and that is what I am doing, at office, home office and home.
They also suggested and I also have UPS backups fro my routers, DSL routers and VoIP switches.
Our Asterisk Servers are protected and our VoIP Phones, a mixture of Cisco, Linksys, Astra, Avaya and Grandstream phones also protected by UPS, over POE.
I even have a Solar Power system at Home and home office. System runs well with most of the time sending excess power to the grid.
But with all these systems, I have lost VoIP service, in two incidents when we lost power in the area, we lost our phone service. But our office and home was well lit.
Why? because the central switch (Belong to service provider) that our DSL service came from also lost power. So only service we had was good old POTS service, and our cell phones, iPhones.
I do not have cable service but I am not sure if they also connect to central switches. And if they do, it will be the same picture, your area loose power, you get no VoIP.
So make sure that you have at least one pots line with a old type phone and well charged cell phone.