Signal for help. All phones have metallic, mirror-like layers behind the screens, perfect for flashing a distress signal that can be seen for miles.
Navigate. Most phone speakers contain magnets, and all phones contain many little wires that can be magnetized to serve as an emergency compass. Only ferrous (iron- or steel-containing) wires will work; ferrous wires are stiffer than copper or aluminum, and are attracted to the magnet. Sweep the magnet across one end of the wire 10 or so times in the same direction (you can also magnetize wire by rubbing it across your hair). Then float the wire in a small pool of completely still water on a leaf or shaving of wood. The wire will align itself on a north-south axis, with the end that you magnetized pointing north.
Make a knife or spear. Abrade the circuit board against a rock to create a cutting edge sharp enough to make natural cordage or scrape tinder. Shape the circuit board into a spear point and lash it to a stick for hunting small game or fish.
Start a fire. Use a thin wire scavenged from the phone to short circuit the battery—just touch the wire to the positive and negative battery terminals simultaneously—and produce a red-hot poker that will ignite dry tinder. Check out the video.
Fish. Use some of the remaining parts to create fishing hooks, lures, and gorges (rather than hooking a fish, a gorge gets lodged in its throat).
Snare small game. Headphone wires can be fashioned into a small game snare.