I recently faced this issue after a recent update on my OnePlus 3. The guide at – http://innov8tiv.com/fix-low-call-volume-oneplus-3t-calls/ is pretty good. However, the “Safe Mode” test failed for me, then next steps was clean with toothbrush.
I was very much convinced that my ear piece was pretty clean and anyway I wiped it with cloth many times and there was no visible dirt. However, when I took closeup picture of the earpiece I noticed that the grill mesh was very very small. And it seemed like maybe those tiny holes in the grill have been plugged by microscopic dirt.
Checkout the grill closeup below (this is after cleaning):-
I didn’t have a toothbrush with fine enough bristles. So instead I used a cotton swab, dipped in nail polish remover. I rubbed with that couple of times and presto, the low volume issue is fixed!
TIP: You need to tilt the handset sideways against the sunlight (as shown in the pic above) to be able to see the grill holes. If they look dark then the hole is open, else maybe they are plugged.