So I recently had this issue and had the hardest time figuring it out when, in fact, it was quite an easy solution. I got a brand new iPhone due to the last one breaking. It is an iPhone 6s. I kept the plastic on it because I was waiting for my screen protector to come. Throughout the next few week's usage I had noticed that I can't hear people when on a phone call. I tried a ton to fix it and finally decided to book a genius bar reservation and had them explain that while the plastic seems thin, and while the plastic seemingly does not block sound, it in fact does block sound.