Mute button greyed out - Any app replacement All,

I have a redmi 1s which I have flashed with cyanogen mod 12.1. All is well except, when I am in a call, mute button is greyed out.

This is a pain for me as I am a part of various conference calls for hours during any weekday and I cant stay without muting always.

Any solution to this in terms of replacement or tweaking my cyanogenmod?

It would be really helpful
Have you tried clearing the cache and data of the Dialler app. If not find the Dialler app under Setting > All apps > Dialler > Clear Cache and Clear Data. Restart you phone and check. 

If the problem persists, then you can try a third party app like Muted Conference Calls