I'm afraid I'm a little confused why you're saying "barometric sensor" or barometer. a barometer is for measuring atmospheric pressure, I'm not sure how that would relate to a Phantom knowing it's altitude. as far as I know, the phantom determines it's altitude with GPS just like it's other positions. GPS is actually a 3 dimensional measurement, not just two. When you power on the Phantom and it home-locks, it registers whatever GPS "height" it's at as ground level and I believe that should be where it bases failsafe at... ground + 20m
Are you letting the Phantom properly do a GPS-lock (rapid blinking green lights) before takeoff? Have you checked in the Phantom Assistant software if the IMU needs calibrating? Have you done the "spin around" compass calibration procedure? If you don't know what any / all of those are I'd be happy to elaborate