this has already been covered, but since i'm a nice guy, i'll tell you from my Z-series experience what worked for me.
here's the link to the other thread that you would've found if you would've searched.
http://www.avic411.com/forum/viewtopic.php?t=15732
I had a Z2 and it was notorious for mic problems. A few things that helped up in the Z-series forums were:
1) when pairing the phone, make sure the volume on the phone is turned less than 1/2 way
2) set your output to come from the left side only
3) dont turn the output for the voice very high, it can ricochet off the other door and into the mic, causing an echo for the caller on the other end.
Try these things and if they dont work, dig around the Z-series forums. Alot of solutions were found and there's plenty of info there.
I have my mic placed at the top of the A-pillar and have no problems at all with my F90BT.