I was also suffering from this problem this solution is only for the people with android mobiles. I tried almost every solution but all in lost. The microphone works when loudspeaker is on but when during a call i turn off the loudspeaker the mic didn't work. As it is a software problem, i tried to figure it out and then i came upon a solution all this is happening because of the Google "Ok Google" detection turned. Simply turn off the Ok Google detection and test. If the problem is still not solved try to Disable the third party apps that you think can interfere with microphone and calls e.g Call Recorder applications etc.
Iphone users may also try to figure out the applications that might interfere with the calls.