Well there was a battle in Florida. I didn't follow in detail so there is some chance my details are wrong.
Several years ago they were not going to allow any IP based phones sold after a certain date because none of them at that time had any 911 service.
Of course this went over like a Lead Zeppelin, without a "Whole Lotta Love".
So a compromise was reached that if they put in their agreement that it would not work for emergency phone calls they could sell them.
That is pretty much the state of such devices still today in Florida.
=======
Now that said. I know people in town that have an IP based phone and pretty happy with it nearly all of the time. But those that say they are happy also say they have a cell phone and very strong service. The chance of both of them going down at once is probably even less than a POTS line going down.
Those in rural areas like myself have to have a POTS line, period. I even tried with omni at various heights up to 20 ft (10 to 12 worked the best not to hear other cells). I tried a yagi that totally stunk and the omni out performed it. Probably something like a panel
antenna or a small dish or corner reflector would probably be the ticket toward a given cell site, but I stopped at not building such an antenna. I tried running a legal amp I bought from Alltel (before selling to Verizon) and it helped but not all the time.
I finally decided that POTS was worth the money.
Then the femtocells came along but only Sprint had them. Sprint has the worst coverage in rural areas in most places including where I live, so even though that would have solved the problem in my house, it would not solve it around town, where only Verizon has any signal (weak) at all.
Verizon was going to put up a limited cell site in our town but backed out. One is the county signed an agreement with them after they said they would not be putting up the site. They knew they would win anyway as they have the best rural coverage anyway. The tower company they were to lease from even erected the tower, but it just sits in our peanut field with nothing but a light on top.
Maybe one day. In the mean time I will pay for a POTS line for as long as I can get one. (unless Verizon comes out with a femtocell device).