Mic is not working when loudspeaker on

[Android Central Question]

My case seems slightly different, If I turn on loudspeaker during a call, Mic will not work i.e person at other end cannot here my voice.
I've this problem in my Asus ZenFone A501CG mobile.

 

[B. Diddy]

Welcome to Android Central! It could be that the secondary mic (the one at the top of the phone, which is often the one used for speakerphone) is faulty or blocked. The secondary mic is also used as one of the stereo mics for video recording, so try recording a video to test stereo sound. Does that channel sound quiet when you listen to the playback?