I can see that a mobile phone may help in the situation where you come across a badly-injured fellow competitor, where time is of the essence, and you happen to have a signal. I'm not aware of any of these cases - have there been any?
I'm not sure how a mobile phone would help in the case of slow-onset hypothermia, where decsion-making faculties are dulled, you are by yourself, and you just want to lie down and go to sleep.
My understanding is that the few deaths in fell races have been the latter.
I may be wrong though?