Can you please help me to understand the below concept using this example?

A cell phone manufacturer inspects the video display on each color phone to verify that the screen can display all colors with the brilliance their customers have come to expect. Each phone is turned on, run through a self-test procedure, and classified as either acceptable or unacceptable based on test performance. The production team takes a sample of 350 phones and finds one with a defective display. How many additional phones should they sample to establish an average fraction defective for their statistical quality control initiative?