I'd imagine floods and earthquakes would be the most difficult and impractical, as there is very little fore warning. In at risk areas, there is some normal level of activity, how to determine a potentially disasterous level in time to be of any use...
Fire would be the simplest, just interface your cell phone to a smoke detector through a microcontroller. I've seen several SMS/microcontroller projects on the web, so that part shouldn't be too tough to get going. Solder a couple of wires to the smoke detector sounder, a resister, and you have your input...
I don't use cell phones (tools of the devil
), so no real interest in how they work, or what you can do with them. Although, I did see some for $1.00 surplus, I was curious about the graphics LCD...