Perhaps I should expand on my comments somewhat. Cell phones are not perfect, and they definitely don't work consistently (especially mine), but more and more often they are the means for initiating an operation. This replaces the classic opener - "ranger, ranger, find my child! - Where might your child be, ma'm? - Somewhere in GSMNP!" Thus the first responders may have an entire park or mountain range to search.

In the cell phone initiated scenario, you know positively that someone is (or thinks they are) in trouble, about where they are located - just vastly more information than we typically had in the "old days." Still better are PLBs. Particularly intriguing are the newer models with messaging capabilities.

The millennium isn't here yet, but cells do help.