Of course, if cell-phone service or radios are simply not operating in your part of the backcountry, you'll need to use other types of audible signals. We're talking noisemakers here, and the type you use will depend on what you have at hand. If you're in or near your vehicle (even if it has run off a cliff into a gorge and you're trapped inside), use the horn. Make three short blasts, then wait a few minutes, then follow with three more short blasts. Sounds grouped in sets of three, whether it's gunshots or blasts on a horn, are a universal distress signal.