I have a personal tale from the Alps from a few years ago - actually, 1991.
I was climbing with a friend on Liskamm, a mighty 4,000 metro peak that looms over the Gornergletscher near Zermatt. Our intention was to climb it and be back in Zermatt for the 1st August shenanigans in the village, which promised to be even more massive than usual, it being Switzerland's 700th year as a confederation.
Things went awry - poor weather and a slow start - and we spent the night in Italy in the Gniffetti hut. The following day, with another slow start, we did top out on the summits of this enormous peak but left ourselves little time to get back to a hut of any description by nightfall. Stupidly, in the interests of speed and therefore 'safety', we were not carrying bivvy gear and so had to find some sort of shelter. But which route to choose? We couldn't return to the Gniffetti easily and the Quintino Sella meant climbing another peak so, reluctantly, we thought we'd try a descent of the Zwillingsgletscher, between Liskamm and Pollux, and aim for the Monte Rosa hut.
It is a tortured glacier with many nasty-looking seracs and, because its descent goes over lots of lumps and bumps, it is heavily crevassed. As the more experienced of the two, I was at the back when my friend took a big tumble into a crevasse. I stopped his - and my - falls, secured the rope and went tentatively to the edge of the dark hole to see if he was OK. Needless to say, he was scared witless and had injured his head. Fortunately, I remembered how to rig a pulley system and, before long, had a really good, secure 3:1 pulley rigged up. I persuaded my friend that he should do what he could to help speed the process and stay warm. With the afternoon advancing all too quickly, I realised that we were in a worse position than before: still no bivvy gear and now with an injured party who would be understandably reluctant to continue a descent of this singularly nasty and threatening glacier.
At that point, I started blowing the International Distress Signal (six blasts then a minute's pause and repeat until you hear the answer) on my whistle while pulling on the rope to get him out. I had no idea whether it would be heard though, in the distance, we could see the Monte Rosa hut and I quietly hoped that someone there might notice the call. Someone did.
As I hauled my friend out of the crevasse, we heard a distant buzz. I started administering some first aid to his impressive head wound when suddenly, the helicopter materialised, dropping off a couple of guides who, in turn, took over the first aid bit and helped me pack up my ropes and belay stuff.
The helicopter ride back to Zermatt was terrifying because the pilot insisted on playing in the thermals rising up near the cliff faces. When we arrived, the doctor was initially uncertain about which of us was injured: the bloke as white as a sheet or the one covered in blood. Sense prevailed in the end.
I learnt a VAST amount from this experience but, above all, I learnt the value of a whistle. I carry the loudest one I can find with me at all times when I am out and about, be it in the woods or on the moors or in the mountains. It is cheap, light and doesn't have batteries that might fail; and, if you're really lucky, there may even be someone listening out for your cry for help.
Sorry to be long-winded and 'out of the woods' as it were but I thought I'd put in my ha'pennorth to the whistle discussion.