When making calls the screen turns off I have Karbonn A9, Android 2.3.6. 

When making calls the screen turns off and comes on pressing the power button. But sometimes it fails to come on, forcing me to restart it by removing the battery. Any clues?
I purchased an A9 today and I'm having the same problem. If you have a screen guard, make sure it is not covering the proximity sensor. That worked for me.