A few years ago, I would have said to forget about using a cellphone in the backcountry since reception was so limited, particularly in the western U.S. That has changed; even remote regions like the Navajo Nation in Arizona have cell-towers. You're still limited if you're in the bottom of a canyon or between mountain passes. That’s where an Iridium satellite phone would come into play. I have used sat phones in narrow canyons and other places where cellphone reception isn’t available.
Even if you don't have reception, your cell phone can alert rescuers to your location. A lost dayhiker in the Superstition Mountains east of Phoenix was found by rescuers after they used night-vision goggles to locate her illuminated cellphone screen from two miles away. In another case in the mountains of Oregon, a stranded motorist and her two children were located by plane after technicians triangulated her cellphone transmissions. (She may not have been able to get a signal, but her phone was periodically "checking in" with a nearby tower.)
If you'll be traveling in a remote area, I recommend bringing backup signaling items such as a sat phone, signal mirror, and ACR Strobe light. If you get lost and your cellphone gets reception, great. If not, keep in mind the above stories and continue to use your cell throughout the ordeal as you never know what capabilities searchers have on their end.