A cell phone at altitude can be 'seen' by too many base stations and it starts to swamp the system. The base stations and channels allocated to them are v carefully thought out to provide the maximum spectrum use - topographics, buildings, output power, anticipated phone density and use are all considered. Stick a phone 5 miles up and all this network planning goes out the window, it'll block whatever channel its operating on on umpteen base stations, possibly hundreds in cities where a base station might be designed to cover just a section of a single street.