After its initial acceleration, the phone probably decelerated as its altitude decreased. The air density at 5 Km is about 0.6 of that at sea level. As drag is

After its initial acceleration, the phone probably decelerated as its altitude decreased. The air density at 5 Km is about 0.6 of that at sea level. As drag is proportional to the air density and the square of the velocity, I think that implies the phone would reach the ground at a speed ~3/4 of its peak velocity (putting aside the admittedly significant issue of its orientation and rotation throughout the fall.)