For freeriding, just to give you an idea,last season i was at the zinal freeride in switserland with some friends, in the days before the contest we had a chance to scout the area with a guide, long story short we did a descent on the backside of the resort, a day later 3 of my friends try to do the same descent but pick the wrong couloir and end up in a village about 2hours walking away from where they were supposed to be....
none of them had a cellphone and we (in the lodge) knew what they were going to do and knew they were much over time, we gave em an extra hour or so, but then we still hadn't seen or heard of em, i even called some hotels/bars in the village where they were supposed to ski to, but nobody had seen em...
i had just called to 112 (emergency number in europe) and they had just linked me to the rescue helicopter service for further procedures when suddenly i see a skibus stop in front of our door and the three goons step out....
2 minutes later and we'd have had a 5000 euro helicopter ride to pay ...
If they had just had a cellphone with them they could've notified us or we could've called them ....
And as far as instructing goes, i give my phonenumber and the number of the organisation to all of my kids, so when they're lost they can either call me or they can go in the lodge and have them call me ....
i feel like it's my responsibility towards their parents to do everything to keep their kids safe and happy ....
and being accessible through cellphone is one of the musts imho ....