The only force acting on the phone is gravity, which causes all objects to fall at the same rate (neglecting air resistance). The magnitude of this is 32 feet per second per second approx. That means, that for every second of fall the speed increases by 32 ft/sec. the phone starts from rest, zero velocity. After t seconds its speed increases to 32t ft per second. Its average speed is (0+32t)/2=16t. The distance it falls in time t is speed times time = 16t×t=16t² so if t is the time it takes to reach the ground 350=16t². And t=√(350/16)=4.7 seconds approx.