I don't think the phones ever work at cruising altitude anyway. I've definitely noticed once or twice when I've forgotten to turn mine off that there was no service when I corrected it.

I only found one good link, but I know I've read other reports that on average several phones are left turned on during every flight tested. It can't be that problematic if this is true. Here's the link for some fun reading:
http://spectrum.ieee.org/aerospace/a...any-airspeed/1

Oh and just to be clear, I'm by no means defending this woman.