Because cell phones don’t have cell service everywhere, when you are out there adventuring you should always carry a device that will let you communicate even when you have no cell phone service. These include the Garmin inReach, ACR Bivy Stick, ZOLEO Satellite Communicator, and similar Iridium satellite communications devices. These are reasonably affordable and have modest subscription costs. It is not necessary to buy and use a full Iridium phone device and those are far more costly to own and use.
These Iridium satellite communication devices, used in conjunction with your smartphone, will let you send and receive text messages even when you have no cell service. This means you can let someone know you need help, or that you are just fine, where there is no cell service. These devices also have a Personal Locator Beacon SOS feature that will summon emergency rescue assistance to your location in a grave, life-or-death situation without needing your smartphone. There are also Personal Locator Beacons that do not allow full 2-way text communication but can summon emergency assistance. Some, like the ACR models, require no subscription, just registration with NOAA. Some, like Spot, require subscriptions.
Whether you have cell service or are using an Iridium satellite communication device, when you need help you should be ready to provide your location coordinates to the people you contact for assistance. These should be in the decimal format, for example: N35.50080 W116.11872. This is the format all North American Search and Rescue groups use. It is also the easiest coordinate format to work with and I suggest you change your GPS devices and navigation software to default to this format.
All smartphones have GPS capability and can display your coordinates, though you will need a third-party app to see them in decimal form on the iPhone. Using software like Gaia or OnX you can display your location in decimal degrees. You can also use an app like FindMeSAR (https://findmesar.com) to display your coordinates with elevation and a location accuracy factor. This information can be texted to search and rescue personnel so they can find you quickly and help. Take time to experiment with this capability and become familiar with it before you need it. Then, don’t leave home without one! Or several.