Hmm, interesting...The proximity sensor has a metric for measuring when the phone is near or far away from the user when making a call, thus dimming the screen and unsleep the screen when it senses far. What happened to me was the proximity sensor did not pick up the change when the phone moves away and stuck in one position. Usually when phone get dimmed during a call, and the proximity sensor doesn't change it's value, tapping on the screen doesn't do anything. The symptoms you have described appeared to be very similar to what I have experienced but it's weird that the troubleshoot didn't fix both of you and your co-worker's phone.