The phone as a signalling device: light, screen, sound, emergency function

Rescuers search with their eyes and ears. A bright point of light stands out at night because everything around is dark. Flashing stands out even more than a steady light, because the eye reacts strongly to change. The best-known light signal is SOS: three short, three long, three short flashes, then a pause, and repeat.

Sounds carry where nothing can be seen, for example in dense forest or fog. A shrill alarm or ringtone is easier for nearby searchers to hear than an exhausted voice. A phone speaker's range is small, though; a whistle is louder and needs no power.

In sunshine a switched-off screen can act as a weak mirror. A real signal mirror is much better, but a flash of light from a phone is better than nothing. With signal, the most important function is added: many phones have an emergency function that dials the local emergency number after a button is pressed several times and can notify stored contacts.

Get to know your own phone's emergency function at home, by reading the settings, not by trying it out. How is it triggered? Does it call automatically? Who is notified? While you are at it, fill in the medical ID and emergency contacts; many devices also show this information on the lock screen.

In an emergency: power-saving mode on, brightness down, unnecessary apps closed. When there is no signal, the phone keeps searching for one and uses a lot of power doing so. It is better to switch on airplane mode and check briefly for signal at fixed times – ideally at times someone knows about.

Answer calls from unknown numbers. If you are missing, the dispatch centre or mountain rescue may call you back, and you will not know their number.

Never trigger the emergency function ‘just to test it’. It can place a real emergency call and tie up rescuers who are needed elsewhere. If you trigger an emergency call by accident, do not just hang up; stay on the line and explain that it was a mistake.

Cold and wet are the phone's enemies. In frost, a battery that was half full a moment ago can suddenly shut down. So carry the phone close to your body in an inner pocket and protect it from rain, for example in a freezer bag or dry bag.

Do not rely on the phone alone. A whistle, a headlamp and a signal mirror are more robust and independent of the battery. The phone is a good makeshift, but no replacement for these simple tools.

You have injured yourself in the forest and cannot walk on. With signal, you call the emergency number at once and stay reachable afterwards. Without signal, you save battery, and if at night you hear voices or see lamps, you give SOS with the torch in their direction.

With a very weak signal, a call sometimes fails while a text message still gets through. Some countries have emergency SMS services, often requiring prior registration – check this locally. A text with your location to a trusted person can help too.

You can hear searchers but your voice is hoarse. A loud ringtone or alarm can then guide them over the last few metres to you. Switch it off again afterwards to save power.

Practise the SOS signal with the torch at home in a dark room: three short, three long, three short. Many people find that a ‘long’ flash can be clearly longer than a short one so that the two are easy to tell apart.

Find out how to reach power-saving mode, airplane mode and the torch quickly, even with cold fingers or gloves. Try out how to set a loud alarm without having to search for it.

Go through your emergency contacts and medical ID once and ask yourself: would a stranger helping you know whom to call? Keep the details up to date.