By following below URL, I managed to resolve it.
https://www.reddit.com/r/Nexus6P/comments/4i2ps4/post_may_update_check_of_the_6p_microphone_issue/
Going through this youtube ->
https://www.youtube.com/watch?v=MzKn0wGpqlA
1) Install Root Browser
2) Open and navigate to SYSTEM folder
3) click on the build.prop
4) Click RB Text editor
5) Change persist.audio.fluence.voicecall=true to false
6) Test the call with your friends, voila ~
