If you're experiencing issues with your proximity sensor read ahead. Solution requires disassembly of the phone and some components and is not an easy job. There's a high risk of breaking your phone or some key components. Please do this at your own risk and only if you have the necessary experience, if not - visit a repair center and talk to them showing them the photo below. I'm not responsible for issues caused by the manipulation.
Your proximity sensor is not working due to bad design of the phone. The proximity sensor is located on the top of the phone, next to the camera, just below the speaker opening. There is no grill inside the speaker opening and dust is free to enter the phone. It's slowly accumulating on the inside of the glass preventing the proximity sensor to properly behave - it's working as if there's constantly something in front of it, which is actually true - there's a large amount of dust. Proximity sensor constantly being blocked results in black screen immediately when you open a call, screen not being activated when you receive notification and double tap to wake not working. In order to fix it you have to disassemble the phone, clean the dust and put some kind of grill between the speaker opening and the speaker itself. You can find some tiny mesh for speakers, it should work but eventually dust might accumulate once again.