Also of note (most people know this, but might be worth sharing anyways) I believe emergency calls get special handling by the network, and can go over any towe

Also of note (most people know this, but might be worth sharing anyways) I believe emergency calls get special handling by the network, and can go over any tower, not just your carriers'. So if you're somewhere with no reception and you have an emergency, try making the call anyways - it might still go through. This is presumably why cellphones differentiate "Emergency Calls Only" from having no service entirely.