I think it's best to be prepared for the worst. My friend who rides alone all the time (and is luckily always prepared) was hurt not to long back while out on trail. She stopped for some reason and got off (possibly to pee) and when she went to get back on she fell and dislocated her shoulder. She was able to phone the barn for help and someone came out and rode her horse back while she went off to hospital.
I always keep my phone on me even when i'm out with a group, just in case.