GPS altitude is notoriously imprecise, even with good units. On a ride through the city when the chip knows where you are to within 10 feet in lat/lon terms, it can be 150 feet off for altitude. That's why Garmins - including the fancy but not brand spanking new anymore Edge 800 - will recalibrate themselves based on USGS DEM (?) at the beginning of the ride.

I don't know of any phones with altimeters in them, though, so it's probably a moot point.