Screen goes off during call and does not come back in Micromax A89
I faced the same problem but I found a simple and perfectly working solution:


Dial *#*#7769#*#* (Proximity sensor calibrator) 
You will see 2 options Start and Exit
Click on Start. 
Move your finger over the proximity sensor 3-4 times and try to change the value on screen.
If values are changing after 3-4 tries then your problem is solved.


Thanks to nighil g