I wish I knew if there was still a signal coming in during a blackout. I have a theory that if you use a UPS to keep the modem, routers, and phones working, if there is a signal, you will have phone.
During that big blackout a couple years ago, the land line phones died eventually (when all the batteries / generators were depleted), and I had issues with my phone for weeks after.
The digiphone people have a disclaimer that says if the power is interrupted you will lose phone service. However a lot of these places run on backup power for quite a while to prevent service outages during smaller power emergencies in normal times.
They won't flat out say yes or no, or they become liable. I guess the only way to know for sure is to see if the setup will work during a real blackout.
I will be going for TWC RR digiphone in the near future. Especially since they offer E911 service, and I don't care for Verizon. I figure I'll save about $20/month and have more services.
As for Cell Phones in a blackout, during a major blackout, once the power to the towers goes, so does your service. Your only option is really a satphone as long as you can power it.