I had digital VoIP phone through brighthouse. Each phone requires it's own modem and that requires in turn electrical power so when my power went out? So did the phones. The upside to basic old school telephone systems is THEY have their own power sources and when power goes down on the lines? The phones stay up.
Not to mention at the time I had to pay an additional 9.95 a year to a service that called 911 for me when I called 911. They've changed this in a few areas, but from what I have heard largely 911 systems still don't support tracing of VoIP phone systems and that's a real danger I won't risk anymore.
In my case, I have a VOIP box that has a backup battery. It does work during a power outage. Granted, in the event of an extended power outage, I might lose service. However, where I live, if the power goes out for that long, the AT&T phones would also go out. A few years ago, SW Bell told my 80+ year old neighbor that she would have to wait 18 days for them to get around to fixing her phone after a big rain storm. My cable VOIP never went down. Sadly, around here, this isn't uncommon. The local phone company service is not very robust. The neighbor learned to use a cell phone.
Also, my VOIP box is plugged into my home's phone wiring. I use the same phones as I did with AT&T. The voice mail indicators on the phones also work just as they did with AT&T. A caveat is that the cable company says they won't support devices like fax machines or alarm systems that use phone lines.
Since my service is bundled, I don't pay an additional 911 fee but I can't recall how the cable company told me they relay address information to emergency phone centers. I'll have to ask them about that again.... I don't think it's an issue here though.