Alan,

in contrast to what the other poster said, I can't see why GSM wouldn't
work up to altitudes of around 5000 feet, at least. It does work here.

As for the cell phone adaptors, just one thought: before getting one of
the expensive ones, it might make more sense to buy a headset with a
cell phone connector integrated, if you don't have the headset of your
dreams already.

--
Thomas Borchert (EDDH)