If you are using that phone with Sprint, I suggest you make sure the phone is set to Sprint PCS (digital) only mode--it's a very real possibility that your phone is switching to "analog" mode while in flight, since it will have a very hard time acquiring and holding a digital cell at that speed and altitude.

I suppose the more important question to ask is why you are leaving the phone on in flight? The airlines, the FAA, and the FCC all want you to turn the phone off during the flight, which should solve the battery drain problem quite nicely.

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Don't feed the trolls.