I flew with a captain last month whose cell phone interfered with the radios. It didn't even have to be in use, although it was worst then. At times, even when it was simply in the standby mode, it would cause static on the radios. I don't remember what kind of phone it was, but it was a newer model.
My Nokia has no noticeable effect as I have "forgotten" to shut it off a "few times."
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I tried to use my handheld GPS on an airline flight once, but was unable to get a signal in the passenger cabin. I usually had to place the antenna on the instrument panel even in GA airplanes.