The rules regarding cell phone usage seem to be fairly clear and direct. The Federal Communications Commission "prohibits the use of cellular phones using the 800 MHz frequency and other wireless devices on airborne aircraft" (see website below.) The belief is that it could interfere with communications capabilities between the plane and ground sources. The individual airlines enforce this in their own manner. Prior to the flight, some airlines allow you to have the phone and communicate on it. Once they begin the process of leaving the gate, though, the phone can be with you, but it must be switched off and should not be used at any point until the plane lands.