Recently I also have had the same thing. Somtimes the mic works during calls, somtimes it don´t. First it only worked slightly, with very bad sound, so the caller couldn’t hear me properly. It sounded as if I was far away. Now suddenly it has improved, but only sometimes. I haven’t done anything at all with the phone. No upgrades or nothing. I contacted the support center, but haven’t heard anything.
Unfortunately there seems to be a problem with the mic on several FP3 phones as discussed
here
and
No sound during some phone calls on the forum