My iPhone X is covered with Apple’s Folio Case and it works fine except the phone call. When I make calls with my friend, the microphone muffles at times so people couldn’t hear me properly. That also happens when I receive phone calls. It sound like that I am underwater. Then I try voice memos and it is fine. I remove case and then on and it makes the same noise。 If I change it to the bluetooth headphones and it is crystal clear.
I do the Hard/Soft Resets, go to Settings app and disable Accessibly “Noise Reduction” but it is still not working. It is not the bug with iOS 11 for that. My firend also face the same problem on the same hardware and the microphone become muffling with call.
If you happen to have your iPhone X with Apple’s Folio case, the issue is caused by the case. A magnet inside the case is used to fold the case back on itself, but it is near the mic and make the call to be muffled. To resolve it, you could open the case so it is not folded back on itself.
However, it is not the magnet and not another antenna-gate. This seems to be a basic physical design problem. it may be a software workaround for that. I think the app can disable the use of the rear mic during phone call, or replace it with an aperture (hole) in the Folio’s flap.