Q: iPhone mic sounds muted

My correspondents complain that my voice sounds muted on my iPhone6s. Sometimes they can hardly understand me. I checked calling my voice mail. Could hardly understand myself

What I already tried to no avail :

- switch off noise reduction

- go to original settings

- reset iPhone

only thing that seems to help is when I switch on the speakers which is ok when you're home alone, but kind of impossible when outdoors or in the office.

And no, there is no plastic covering the mic and no I do not keep my hand, fingers or cheek against the mic.