It's not impossible to make cell phone calls from altitude, it's just not recommended. The official reason is that a single phone can tie up many relay towers at once as many more are in range than are on the ground. Also there are some concerns that a cell phone signal may interfere with cockpit instrumentation. I don't know if they have ever seen this happen but I have seen home computer monitor get fritzy when cell phones ring and many hospitals ban their use for the same reason.

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------------Mark Twain

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