Hi!
I’m very happy with the level of the audio on my FP3 during calls through the “ear” speaker (not the main speaker) from the day I bought it, a year ago. I’m using the original Fairphone 3 (not the FP3+ version), so I don’t know if this is common problem with the “Top+ module” or something, but I don’t remember to read about this issue in the forum (nor with the old module). Maybe it is a probel just on your unit, or maybe is a software bug related to the recent Android 10 update.
Anyway, and just to clarify it, I can adjust the volume of the “internal” “ear” speaker via Settings -> Sound:
I’m sorry I can’t be of much help. I wish you FP support will answer you soon. Best of luck!