Well, first of all, people with issues are the ones that create topics here. People that don’t just happily use the phone so you won’t see/hear them. Sadly no phone manufacturer produces a fully functioning phone, especially in the first 6 months after launching the phone.
In this thread you can find some user experience. This problem didn’t really occur with regular phone calls, mostly with other apps.
Fairphone’s goal is to improve the microphone quality.