Ans. Each phone has a proximity sensor to turn off the display when you place it on your ear during a call. This prevents accidental taps on the touchscreen by your face. The proximity sensor is activated everytime you place a call. It works by detecting any solid object that comes near to within 4-5 cm of the phone screen. In the Nexus 4 the proximity sensor is located at the top left corner of the phone above the screen. Sometimes if a screenguard/scratchguard or any phone cover comes over the proximity sensor, it will lead to the exact same problem that you are facing now. Please ensure that nothing is covering the proximity sensor. If nothing is covering the sensor and even then you are facing the problem then you need to take your phone to the LG service center as your proximity sensor has developed a fault.