Lots of the apps are free but you first have to make sure your existing device has built-in accelerometer and gyroscope features. That’s what allows it to detect falls. Check your phone’s specs-- if it has an accelerometer and gyroscope, then it almost certainly has a fall detection app already built-in. If you have an older phone, and this feature is important to you, maybe this is your excuse to upgrade to a current model.
But, it doesn’t have to be a phone-- many of the current GPS watches have this feature too. You could get a used Garmin 45S on ebay for about $100. The garmin watches don’t directly call 911, they call your pre-determined emergency contacts, but the advantage is the watch will provide them with your exact coordinates. So even if you’re out on a trail and unconscious, EMS would be able to find you.