The Ooma device is pretty good for this purpose for us. Once you buy the device ($200 some years ago...under $100 now), the basic service is "free" -- you just pay the monthly government taxes and telecom fees of about $5, and calls within the US are free/unlimited I think.
Main point being that you have something that does not rely on any one fiber, or any cell provider.
I use a company called VoIP.ms for service. It's measured, but it's $0.85/month. That's connected to a SIP adapter (Grandstream HT801), and that has a phone jack, and an ethernet jack. The adapter supports pulse dialing, so I have a vintage rotary phone (it was a gift -- THANKS!).
I've seen what UIA has for backup power, they're staying up. On my end, I have a good UPS for the internet/WiFi.
There are less technical solutions, but this was fun to put together.