When I first started hiking, the idea of a cell phone we could take hiking was so futuristic we never even thought about it.  At that time, cell phones were only in cars, and if they were mobile, it required such a big and heavy battery pack and antenna that it wouldn't be practical to bring into the back country.  Also, at that time, all they did was make phone calls, not even text messaging!  But technology has changed and now we have powerful little computers in our pockets that do almost anything we would want, including that rare event of making phone calls.  So, how do we use this technology in the backcountry?

First, we have to worry about battery life.  Phones, especially older phones or phones running many open applications, run through battery charge quickly.  When you add in cold temperatures that you find in the winter, and they die even quicker.  So, you need to be conscious of what your battery is going to do.  The easiest things to do to save battery are to put your phone in airplane mode and to close all unnecessary apps.  With airplane mode, you can still receive GPS signals, and if you set it up right, you can still talk to your Bluetooth accessories like your watch.  Turning on airplane will shut off the cell and WiFi radios and stop them searching for signal, which is typically the biggest drain on your battery.  If you're finding that even with that your battery drains quickly, you may be in need of a newer phone.  My experience is that 2 years is about what you get from a phone before you start having battery issues.

What do we need the phones for then?  Well, that's a very valid question to ask.  The biggest uses would be for GPS and to take pictures.  Some people try to use them for flashlight, but that only kills the battery quicker, and doesn't provide a good light for hiking.  A dedicated flashlight or preferably headlamp, is what you should carry as part of your 10 essentials.  Back to phones, they are delicate, so we shouldn't depend on them exclusively for navigation.  Our battery could die along with any backup charger.  We could drop it breaking the screen making it unusable or dropping it into water killing the phone.  We could lose it somewhere on the trail.  For all those reasons, you really need to be comfortable with using a map and compass for navigation.  Yes, the vast majority of the time you can rely on your electronics, but be prepared for that one time you can't.  As a side note, when charging your phone on the trail, it is much more efficient to use a cable instead of wireless charging.

How to take photos and videos on your phone should be quite obvious to everyone, and I highly recommend you learn how to open the camera using button pushes so you don't have to unlock the phone to take a picture.  That said, be conscious of where you are and what is around you when you're taking pictures, especially selfies.  People have fallen off cliffs for lack of awareness.  

The other major use, GPS, has some things you need to consider.  The app you use is the biggest question.  The big ones out there are AllTrails, Stravia, and Gaia.  I personally use Locus Maps, and CalTopo.  There are others out there too.  It's all a matter of what features you want and how you're going to use it.  There are some universal things though.  First, plan on not having any data from the moment you park until you get back to your car.  You may be lucky and have a strong signal the whole hike, but don't count on it.  Before you leave home, make sure you have downloaded the map for your hike.  You should also have mapped out and downloaded your plan B hike if you can't do plan A, and any options for extending or shortening the current hike.  Mapping changes in your route typically are not easy to do when you don't have a data connection.

The next thing to think about is when you download the map, does it show you all the trails in the area or not.  Test this out before you go on a hike.  You may be able to turn off cell signal on your phone, start the hike, and see if you still see other trails, or just what you have mapped out.  If you don't see any of the other trails, you may want to consider upgrading to a paid version of the app where you can see all the other trails.  Knowing where the trail junctions is very important when hiking.  As a group, you want to stop at each junction and make sure everyone goes up the same trail.  There are plenty of stories where people take the wrong trail, don't know it, and now they're lost.  If you're not recording your hike, you won't be able to backtrack.  If you don't see all the trails on your map, you may find yourself lost.  Apart from being able to download all the trails, purchasing an app subscription may bring other benefits like being able to plan hikes on your computer where it is often easier than trying to do it in the phone app.

My personal experience is with Locus Maps and CalTopo, and they have their uses.  Locus is a lot like Gaia and AllTrails.  The paid version gets you access from a computer so you can do all your route planning using a mouse.  What's nice about this software is you have to buy the map, and that's relatively cheap overall (the map purchases for multiple states are basically included if you pay for the app since they give you so many tokens for that).  In the US, you buy the map by state, and you get all the trails within the state.  You can then choose to use the online map or the downloaded map on your phone when hiking.  The planning of hikes online is as easy as clicking on landmarks and it chooses the optimum route, usually opting for the lower elevation gain, but you can either drag the route to where you want, or add in other points to get the route you desire.  There are other nice features in the app like distance rings so it's quick and easy to estimate how far you have to go (as the raven flies).

CalTopo is a bit more of an advanced software.  Mapping of routes are not as easy as it is on other software, and I find it easiest to map something out in Locus, export as a GPX, and then import it into CalTopo.  The best features of CalTopo is that you can create PDFs of the area you are hiking and print them out.  You can also purchase custom maps from them with all the features you want, paths colored in as desired, and at the scale you want.  Not everyone on your hike may want to pay for a map of the area, but they most likely can print a PDF somewhere, and use that map.  CalTopo is also used for Search And Rescue services, and they provide a rescue mode to help with grid searches.

Ultimately, before you go on a hike, make yourself familiar with the GPS app you're going to use, and know what the limitations are, and plan to overcome those limitations with other methods.

Newer phones are now starting to have emergency satellite communications.  This is at the moment usually limited to text messages, but I would not be surprised if they eventually allow images and eventually voice.  This is a relatively new feature of phones, and is not universal at the moment, so we really don't know how this will play out in the long term.  The issue with relying on this for emergency help is the same for using a phone for map.  What happens when the phone fails?  Instead, this is one of the few times where I will recommend a single purpose device.  Using a satellite device with SOS capabilities is by far preferable.  Yes, you can use your phone if you have enough battery and are in a place where you're not going to break it, get it wet, or lose it easily.  But you will be better served in an emergency by having that dedicated device.

First off, the dedicated devices tend to be much more rugged so they won't be damaged as easily.  They tend to use much less power, so their batteries last days instead of hours.  Their operating temperatures often are larger than phones too. I have had my phone shut off because it was too cold while my InReach device never had an issue.  The most important thing though is that you turn on tracking so that people back home can follow you.  If something happens and you fall unconscious on a hike, you won't be able to use your cell phone or the SOS function.  If the device is turned off in your pack, it's useless to you.  If you're tracking, your trusted adult that should be following you and that you have calling in the rangers if you don't check in by a certain time, they'll be able to point the rangers to where your movement has stopped.  

The rangers can see the map, they can contact the tracking company and get exact coordinates, and they can focus the search where you are likely to be, possibly down to being able to walk right up to you as they have your device within a 10 foot or smaller area.  This will make any searches more likely to be quick and in the event of the worst, bring closure to the family much quicker.  That said, you still have to be prepared for it to take 2 hours per mile of trail for them to get to you, it's never going to be an instantaneous response.  The flip side of this is if your hike is running much longer than expected, your responsible adult can see that you're still moving, and not needlessly contact the rangers.  How you handle that is between you and your trusted adult.

There is one other thing that I see phone being used for, and this is something I highly discourage.  That is using it to connect to a Bluetooth speaker and playing music.  Often, the music is loud enough that you can hear it many minutes before you come across the person.  If you're both hiking a similar pace and in the same direction, you could be stuck listening to someone else's music for extended periods of time.  Music is extremely subjective.  Not everyone likes every kind of music or artist.  It can scare away all of the animals, lessening the experience for other hikers.  It is just impolite to force everyone else to listen to your music.  If you are one that has to hike listening to music, please use earbuds (not full headphones).  I recommend ones that loop over the ear so that they don't fall out easily, and to use one ear at a time, doubling how long you can listen to the music and also allowing you to be aware of your surroundings.  Please, remember Leave No Trace principle #7: Be Considerate of Others.

In the end, I don't want to leave you with the impression that I am against phones in the backcountry.  I use my phone for photos and GPS all the time.  I am pointing out the limitations, and things I have seen or heard about over the years.  Phones are great, but for everything you're planning to use it for, especially the important functions of GPS and getting help, you should have a non-phone backup and the knowledge of how to use it.