BIG WORD OF WARNING WHEN USING CELL PHONES AS SAFETY DEVICES!!!
They rarely work in the back country.

As a ham radio operator, I know a thing or two about RF, cell phones, sat phones, etc. .....and fell into my own trap when trying to use texting to report positions. The story:
A couple years ago, I did a solo motorcycle ride up Old Ore Road in Big Bend National Park. My friend stayed behind in camp. I agreed to text my condition every few miles. Both our phones showed service at some level the entire ride. Having a great ride, I sent about 10 texts, and tried to call. The texts looked like they went through. The calls went to voice mail. Finally I receive a text from him saying "haven't heard from you in three hours. One more hour and I will start search." I high tail it back and find him loading up the truck to head to the ranger station.
His cell service was Verison, mine ATT. He received all my texts the next morning!

So, in my experience, amateur radio (or a dedicated radio service), sat phones, SPOT (sat based service), will likely work. Cell service should be only considered a convenience, not a solution.