Thanks for the circuit suggestions.
As far as how high a cell phone will work, well, I've looked into it, and I believe it will work. The antennas on GSM cell site are horizontally aligned, and performance from above would be reduced, but they can still pick up signals from above because there are absolutely no obstructions. In fact, the phone would no doubt be hitting multiple cell sites simultaneously, but that shouldn't be a problem either. Take, for example, all of the calls on 9/11/01 that were made to loved ones while flying at 10,000 feet and ~500mph. They must have been hitting multiple cell sites, and hopping sites at a very rapid pace, but the calls went through, and there were quite a lot of them. On a balloon gently drifting up, it should work great, and even better if I use a directional antenna pointing down. There is at least one high-altitude balloon that was launched with a cell phone for returning data and pictures, and it was reliable to the maximum height of the flight, which I believe was 93,000 feet.
If I could remotely trigger the phone to snap some pictures and text them to me, that would be neat, but probably won't see anything but blackness over the Everglades at night.
What makes you think the phone won't work?
