Dude, how long do you think your landline will work? ABOUT THE SAME, unless it's a traditional copper POTS landline. Cable/Fiber gateways have batteries too!
Exactly Dude POTS hard wired to the CO is best for 911. It can survive until the diesel generator runs dry. Like I tell folks at my point in life when I call 911 I want to know the FD knows EXACTLY what door to break down if I can't answer it. If you think cellular can do this you are delusional.
Not traditionally, but I'm pretty sure this system can - not positive but I THINK they're registered to your fixed address since they're marketed as landline replacement...
Now, as for 911 - no doubt, hard wired copper POTS is best. But it's also gone the way of the dodo bird and is getting replaced with fiber. Fiber and cable do no better than these systems for battery life.