I could be totally off base here, but this is around when the first cell phones really started appearing.. my understanding was that since radio is line of site there was a concern that a cellular phone at 35K could potentially ping dozens of towers at once (hundreds?) and cause some issues as a result of that
I think that's been solved by angling the beams more effectively. Full disclosure, outside of some examples, I would say 75% of the time I'm over 4K my phone has zero signal
On a side note.. on more than one occasion I've heard Mexican radio while airborne on the lower 118.XX frequencies in the San Diego area. At first I thought it was a stuck mic on some dude's plane, but learned that it was Mexico just blasting messy radio signals at super high power with abandon into oblivion. A little Googling revealed that this is an issue for Verizon and other customers living along the border