Are you using a phone case by any chance? Have you tried to clean the proximity sensor to see if it's any better? Feel free to visit this link: http://koo.do/1RAyG8b for a specific support if you need help to troubleshoot the proximity sensor. You can also visit a Koodo store nearby so that a representative can take a look at your phone. Please click here: http://koo.do/Kwarranty for more information about the manufacturer's warranty.
The manual does not appear to have anything on the proximity sensor. The diagram of the phone at the front does of the manual not show it. There is nothing helpful on this site but although the case was sold and installed at a koodo kiosk, I will take the case of and see if it makes a difference. Thanks