Screen goes off during call and does not come back in Micromax A89

I am using Micromax A89 with ICS 4.0.4 512 MB RAM Screen 4.0 Inch I am facing this problem since beginning. But it was solved automatically. Now again it happens when I dial a call or receive a call my mobile's screen goes off and does not come back. I must have to use power button to end the call (I set "Power button end call" checkbox from settings). I think it's a proximity sensor problem but not sure? But when I restart my mobile then it works for 1 or 2 calls but after it again it creates this issue. Any help would heartily be appreciated.