I had the same issue with my fathers iPhone X.
i bought a wallet case from eBay named Dulci.
Since day one the quality of his voice was terrible.
I tried several ways to resolve the problem.
Of course I tried the noise reduction ON or OFF but nothing.
Then I tried to remove the case when I had active call, and like magic the issue disappeard.
For helping you I have to mention the case does not cover any mic or any hardware but produce this issue without any visible problem.
I also switch to ROCK case during active call and with ROCK case on it there is no issue.
Now with my X i use not too much I hear complaints about the won’t hear me but not often.
I used two different cases one for ROCK and the other from Caseology Coastline series.
With Caseology I have the issue and after switch to ROCK no issue, but I have to say this is not a primary device.
After a week I will use the X as primary device and I will have a clean image about the issue.