I do periodically consider getting a "tough phone" for the outdoors, but always wind up thinking "Naa" and just putting my Nokia 6310 in a pocket.
If I'm going to be out and about for several days, then I might, like Rik, put a spare battery (doddle to change) in a plastic bag.
I've had the 6310 since it was new and an upgrade from Orange, and it seems to have a standby life of forever and can get a signal in the strangest places. I once received a text from my wife when I was standing on a frozen lake when I was on the Woodlore Arctic course north of Kiruna! God knows where the cell tower was!! That was after I dropped it in a snow drift and took 10 minutes to find it.
I've been told by very young (I'm old) salesmen in mobile shops to hang onto it as it is now a classic!! I usually wind up selling the phones that I've been given as "upgrades" (they all seem to be worse) and just keep using the 6310.