All OS Lenovo K6 screen going blank when calling

Just bought this new phone 1 month back from Flipkart and when I try to call anyone I am able to hear calls and receive calls but while doing so the screen grows blank and hence dont know what to do. Can some one guide me on what should I do regarding this?
 
I had a similar issue with my Mi Max around 8 months back. The culprit in my case was a misaligned screen protector.

Is your screen protector obstructing your phones proximity sensor in any way?
 
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