For those wondering about returning the unit to Pioneer - I tried that to no avail.
Although this seems to work fine for some people I have the muffled sound problem which also means lousy voice recognition. I have tried everything in this and other forum entries, phones, microphones, location, 'OK' button, bits of tape etc. The only test I have not been able to do is try the unit in a different vehicle (it's in a 2005 Honda Pilot). I get much better call quality if I just use the speakerphone built in to the phone.
I sent it back to Pioneer and they returned it about two weeks later