Hi Shashank1971,
Welcome to the Nokia forum.
If your voice breaks up or sounds muffled during phone calls, there's a big chance something's wrong with the microphone. Please visit a local care centre where they can check it out and send it for repair, if necessary. You won't be able to easily fix this yourself.
Go to your local Nokia web page and click on the Support tab to find your nearest care centre.
Hope this helps!
Iris9290
If my post has helped you in any way, please accept it as a solution or click on the white star, so that other users will be able to benefit from it too. Thanks!