Here's a little tip if you're ever in a horror movie.
Your cell phone will not work.
It doesn't matter if it's the beginning of a movie or the middle or the killer's right outside your door. You will at your magical device and see a nice little message that says "NO SIGNAL". It. Always. Happens.
I imagine a room full of horror movie writers watching the news, going "Woe to the maker of the cell phone. Now every single plot we think of shall be thwarted the moment the main character hears a twig snap, he or she pulls out the cell phone, and the army surrounds Jason." So the solution is to make the cell phone never work. This also applies to the GPS and computer. Is there any horror movie where the cell phone works without difficulty. Or the characters are smart enough to use it.
You writers, you're damn lucky that there's still a lot of places where cellular signal is miniscule - forests, deserts, mountains. But mark my words. Soon satellites will cover the world. Soon the words "NO SIGNAL" will be something we laugh at like 5 1/2 inch floppies and TVs without remotes. Your clock is ticking, fellows.