It’s only during takeoff and landings. Also, It’s been about 8 months since I’ve been on a plane. It’s a good thing too, because I was inching closer and closer to saying something that would get me on the no-fly list. ;-)
Last flight I was on (April 2014) the flight attendants announced that phones had to be switched to airplane mode prior to takeoff and remain that way until after landing. They did not require them to be powered down.
In the past 8 months, the FAA has changed their guidance. Most airlines are no longer requiring passengers to turn electronic devices completely off. How, they do ask you to put it in airplane mode.
Leaving the cell-phone enabled can interfere with both navigation and communication, as the frequencies are close, depending on harmonics. Depending on where you sit in the plane, you may be right over the antennas on the belly, or interfering with the cockpit audio system.
Plus, you are just running down your battery faster, as your phone boosts the transmitter to maximum power in an attempt to contact the cell phone tower on the ground (which can't hear it anyway).