On Friday, May 4, 2018 at 6:19:45 AM UTC+1, Mike Schumann wrote:
However, I think relying on cell phone coverage for any kind of
collision avoidance is a highly questionable approach.


This is correct.

Cell towers use antennas with supressed vertical patterns. This gives more "gain" to ground based phones, and improves range.

I dont see this ever changing, and it is a fundamental limitation for airborne use.