I had heard many years ago that hitting multiple cell towers is one of the reasons why cellular calls from airplanes are not allowed (I think the other big safety reason being possible interference with the onboard avionics). At 30,000 feet, you can be line of sight to many towers at once. However, one thing that I never heard mentioned after 9/11 is the use of cell phones by the passengers on United 93 to call family and loved ones. I don't recall ever hearing any mention of reception problems. I have heard of problems with the onboard "air phones" during that incident but not with any cell phones. Perhaps a protocol had been developed by then?