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"On the return flight from Akron/Canton to*Phoenix the first leg was to Atlanta.* I left the Phone off until we started to let down into Atlanta.* Since the weather was good my phone was on from about 5000 feet above ground to touch down.* I had absolutely no signal until we were about*to an altitude of 500 feet AGL (Above Ground Level) then the signal strength went from one bar to seven bars in less than 10 seconds.* This was conclusive proof for me so on the last leg from Atlanta to Phoenix I left the phone off until touch down 'cause there is no way in hell you can make a call in an airplane from a personal cell phone if you are above 3000 feet."
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Mine works fine, but in the aircraft I normally fly in it's quite a bit lower and slower. What you're dealing with is line-of-sight distances with devices that "hunt" for the strongest signal to establish a realtionship. You can be several miles from a tower (just ask the guys who call to get rescued off mountains).
Depending on where you are, cell phones will work at higher altitudes and speeds, but in crowded municipal areas, the tower hand-offs are happening so fast that the network load would be a serious problem if they let thousands of people in airplanes make calls. And even having your phone on generates heartbeat traffic as it moves.
A CDMA system uses power-level adaptation, which means as the number of connections increases, the transmission power is decreased. If you've wondered about dropped calls, consider the concept that "only the strong survive." If you're in a very high traffic area, signal strength from inside an aircraft may be so weak that you won't get service at all.
What you're also confusing is the speed at which that rather crude "strength" display can convey what the constantly-changing signal strength is as you move rapidly from cell to cell in crowded areas.