Issue with Proximity sensor on Xperia M

[Hit]

While talking on phone, the sceen sometimes does not goes off, and lot of buttons gets pressed, sometimes, wifi gets on, sometimes call goes on mute, and sometimes even gets disconnected. 

I have just started using the mobile just 3 days back. Seems there is software issue with PS (may even be a hardware issue, but that changes are less.)

Any suggestions / help , or similar problem faces by any one to resolve this, will be great help.

Thanks, 

[Uliwooly]

Does your phone has a protective case and/or protective film or anything covering or partially covering the proximity sensor (dirt, grime, etc)?