Cell phones are not the best device for emergency communication and cannot replace a medical alert system.
Coverage is not universal, and if you and you phone part company or the phone is damaged, you are out of luck. (If you find yourself someplace with no bars on your phone, you can try to call 911 anyway. Other carriers may pick up your signal and will alert first responders.)
Perhaps the most useful device is a smartwatch with alert capabilities. Gadget Review is an online site that does just what the name says. This is its evaluation of the best smartwatches for seniors: https://www.gadgetreview.com/best-smartwatch-seniors
ROADiD: This is a wrist bracelet and safety identification tag. You enter vital contact and medical information into their database. A telephone number and secure code number on the bracelet allow emergency or medical personnel to access the information. The ROADiD app provides real-time location sharing and accident sensing. www.roadid.com. (FB – ROADiD). It has nice articles about motivation, staying safe when out and about – mostly for hikers and bikers, but the basics apply to us. Also frequent discount offers.) Note to dog lovers: a portion of each sale goes to 4 Paws for Ability, an organization which trains service dogs for children and veterans.
These medical alert systems are highly rated by the impartial USNews and World Report. Their in-home range is between 1,000-1,500 feet. However, they all offer cellular and GPS options which provide greatly expanded range and tracking service. This chart gives an overview of each service for comparison as of December 2021. https://www.usnews.com/360-reviews/services/medical-alert-system.
One Call Alert
Medical Care Alert
Bay Alarm Medical
Lively (formerly Great Call)
Personal Locator Beacon
If you ride in more remote areas where a rescue would probably involve SAR (Search and Rescue teams), you might consider a Personal Locator Beacon (PLB) or satellite locator system. These locator systems are like the medical alert GPS systems on steroids.
This article by REI Outfitters explains them in detail: https://www.rei.com/learn/expert-advice/personal-locator-beacons.html
Personal Locator Beacons (PLB) are used primarily by military and government agencies. They are popular with people who just want to send an SOS in emergencies. You must register (for free) with NOAA SARSAT (search and rescue) to be included in the system. In an emergency, you activate the PLB which sends an SOS and alerts search and rescue teams to your location. The systems do not allow you to cancel the alert which can result in fines and possible legal consequences if it is a false alarm or non-serious situation.
The other option is a Satellite Locator System. These systems allow two-way communication so that you can talk to the dispatcher and rescue crews. Some systems also connect to smart phones which allows both voice and messaging. Others offer additional features like navigation. All satellite systems require a subscription, either monthly or annually.
Gearlab, an impartial review site for outdoor recreation products, uses real-world testing to evaluate products. This link is their rating for both PLB and satellite systems in September 2021. https://www.outdoorgearlab.com/topics/camping-and-hiking/best-personal-locator-beacon (One of the system, SPOT, also offers roadside service with www.findmespot.com. It responds to routine problems for tow vehicles, trailers and RVs. However, it does not service horse trailers if animals are on-board.