I switched to internet phone a couple years ago, its not IP based but at a lower layer over the same cable as my internet and TV. The cable modem has its own battery then I have it and my wireless router and cordless phone base on my own ups so i have several hours or run time.
Compared to a land line I've found it to be way more reliable, I've had cable internet since the 90's and up until Time Warner was AOL'ed never had an issue. Once they merged with AOL and ruined their service I switched to a smaller company called Wide Open West and many times have used the internet to report my land line phone or power was out, the internet connection kept working through various storms or power outages that the other services didn't.
Out land line service was bought out by AT&T not too long before I dropped the land line for internet based phones so thats partly why the land line was so unreliable, seems they have the same issues with wired phones that they do with their wireless