Long time lurker here who registered to respond to this thread.

One thing to note is that sometimes a 911 call might go through even though your phone shows no reception. My provider is T-Mobile, and they used to share cell towers with another major provider (not remembering which) and now, even though they no longer share towers, it appears they have an agreement concerning emergency calls. When all reception is lost the phone will say "emergency calls only". Customer service confirmed that it means you will still be able to dial 911 even though other calls won't go through.

Since I have an older phone mine doesn't display their handy little message, so at this point I just have to take their word that if I need help my phone is going to work. But I have seen it on the phones of those I share my plan with, so I know it does occur.

I believe this information is important because in a survival situation you'd want to try your phone anyway, even if it shows no reception, and that little bit of knowledge could be the thing that saves you. Of course from what I understand it wouldn't work if you're out of range of all cell towers, but at least it's something to try.