Some HTC Desire 616 users have been facing proximity sensor related problem where the backlight turns off whenever one places or receives a call and comes back again in between few seconds of interval.
Fortunately it is only a software issue and not a hardware one. To fix it, go to -> Settings -> Display -> Daydream. In daydream, select "While Docked" and then switch off the Daydream. This problem will be solved.
While this solution is a bit weird, it does solves the proximity sensor problem being faced by HTC Desire 616 users.