I started putting luggage tags with both my home phone number and my cell phone number attached in an obvious place on my saddle.
Luckily I've never had to use it.....yet, but before I started doing that (about a year ago) my friends and I had horses run off and we had to go walk them down and luckily found them. (They weren't from riders falling off but rather horses getting loose at a lunch stop or getting off to pee and dropping the reins and having the horse take off for home......yes, I've had that happen!).
So after several close calls I started putting luggage tags on my saddles. Then if someone finds my horse they can call me. Once, the time I got off to pee and the horse took off for home, my friend's hubby picked me up on his ATV and we traveled the road home and found my horse tied up to a tree. Someone caught her and tied her up on a street corner, which was very kind of them. She had almost made it home too.
Also, as someone else said, keep your cell phone on YOU. It will do you no good if the horse runs off with it. My friend did that once. I had my phone on me but she had hers in her saddlebags. Both horses ran off at a lunch stop and we got separated looking for the horses and I couldn't reach her by phone because her horse had the phone and he wasn't answering.
I ride alone quite a bit and I always tell my family when I am leaving and sometimes they call and check on me. I feel better that at least if I fell off and was injured someone would come looking for me eventually. Or better yet, I would be able to call for help. Now telling them my exact location might be a problem because I am not on marked trails. But some help is better than no help. I pray I never need to test out their tracking skills.
Knock on wood, but the times I HAVE fallen off, the horses have stayed with me. They might wander a few feet and find a bite to eat, but they haven't left me for dead.......yet. And the longer you own a horse, the more likely they are to stay with you I think. I have had a couple of horses that I swear would leave me for dead but then after owning them for many years and then falling off they have stayed with me. Good horsie!