Jim Macklin wrote:
It is an FCC rule because cellphone tower are designed to
pick up a limited number of calls and at altitude the towers
are over-loaded.

Well the issue is that in analog cellular there are a finite
amount of talk channels and they use spatial diversity and
adaptively reducing power to reuse the channels in a metropolitan
area. A plane at altitude even at minimum power is heard equally
well over a wide number of base stations.

Of course, it's more involved now with digital modulations.