Why mobile screen become black ( off) while making or answering call?

[Vilaspk]

I have micromax canvas infinity mobile.
Since last some days I'm facing a problem, whenever I get incoming calls or make a call suddenly screen become black (off).

 

[B. Diddy]

Welcome to Android Central! Does it only turn black as soon as you answer the call or make the call? If so, then something is triggering the proximity sensor (which causes the screen to turn off, since it thinks your phone is up against your ear, and wants to prevent unintentional screen touches by your face). It could be a poorly fitting screen protector or case. Make sure nothing is covering the proximity sensor, which is right near the front-facing camera.