I use a Spot1( got it on eBay for a very reasonable price) so my wife and daughter know where I am. Around here many places I like to hike in don't have cell towers, so phones are not that useful. I can send a I'm ok every few hours on the spot so they don't fret over me.

They are very reliable if you have an unobstructed view of the sky. In deep canyons or under a heavy canopy of trees they are not reliable and very spotty at best. Pun intended.