When airborne you are in line of sight with too many GSM masts and I've heard that this may cause problems because your GSM is trying to contact too many masts at once. Overlapping frequencies, that sort of thing. Apparently to the point where you might bring a whole area down.
I suppose it's possible that if you're high enough and your operator isn't too clever at frequency planning there will be more than one cell within the magic 35km on the same ARFCN.
But ... the cells will have different CGIs and BSICs and so on, and the protocols
are designed to cope with this sort of thing, so from what little I know about it (I know the protocol layers but not the RF theory) I don't think there would be a serious problem. For example, your phone should register with the nearest suitable cell, and should then wind down its transmit power to be sufficient for communication with that cell, at which point I would hope it is not causing a problem to any more distance cell on the same ARFCN.
Mobile phones in a light aircraft
do interfere with the VHF COM radio, so it seems reasonable to assume that they also interfere with the VHF NAV radio and make sure they're turned off when relying on a VOR for your safety.
if a pilot cannot do a left/right roll within 20 degrees, completely safely, his PPL should be burnt on the spot
I expect that I could do exactly that. But it's not part of the standard PPL course, and I prefer to try new things with an instructor before experimenting on my own. Particularly at 60kt and 500' above someone's house.