Doesn't really matter if you leave a phone on.. It pretty much won't work 99% of the time in flight anyway.

Modern cellsite sector antennas tilt down to cover the ground, not upwards to cover the sky. It's slightly different to the old AMPS analogue days where signal would still bleed upwards from omni antennas and some sites were specifically designed for this coverage - Air NZ even used to have a phone on board most of their 737 fleet in the 90s for passengers to use.