It’s not constant or even regular (for me at least): Some days (weeks, even months) call quality is perfect, and then all of a sudden it is abysmal (and at the same locations, using the same radio towers).
It’s clearly a very convoluted issue which can’t be reduced to a specific provider configuration, 5G use, or some such. There are clearly a lot of factors contributing to this happening (for me at least. It’s not impossible that we have here several similar but not identical issues, all affecting call quality for slightly different reasons).
.
Anyway I don’t think the “Phone” app is the culprit, given Google updates this at least once a month. If there was a problem in there it would had been fixed (voluntarily or by accident) long ago. The phone app of today has little in common with the one our phone used a year ago.
My prime suspect (because it’s the only thing we all share and which hasn’t been updated since ??? (ever?) is the phone’s radio chip driver. Which depends on the manufacturer of the radio chip, on whom Fairphone probably doesn’t have much influence. That would explain why nothing has been done all those years. 