Thoughts..
Folks are well known for not doing what they are told - I'd rather expect something more stringent than "you must turn it off" if there was a serious danger? I would presume however that the effect is cumulative, and people being good keeps it to a managable level?
You're more likely to cause a spark by wearing a nice nylon shell suit than using a mobile - perhaps shell suits should be banned (PLEASE!!)
_modern_ mobile networks connect to multiple cells as a matter of course, it's how they do geo based services. Not a problem to the network. I can't comment on older analog phone systems which (at least 3-4years ago) were quite common in the US, and I'm not current on whether they've caught up!
Can you really get a signal that far up? Every mobile cell antenna I've ever been involved with is very much ground plane polarised.. I tend to find a vibration in my pocket as I descend into circuit is indicative of having forgotten to turn the thing off - it rarely if ever buzzes in flight.. (private pilot..) Which makes the whole thing a bit irrelevant anyway!