n5296s
Switch off your battery saving mode, if it is possible on the phone. The handset gets a good signal, so it reduces its own output on the assumption that you are near a base station, but you aren't! I have done this from the Brecon Beacons in South Wales, having hte same problem, and it worked.
IO540
Did you pay attention to your groundschool? 1.25 x square root of height in feet? And what type of aircraft are you flying in? OK, not the 160 kts TAS I usually flight plan, but most travel at 120 mph or so, enough to get a long ban even on the motorway! How many people phone at such speeds in cars?