ZOOM PHONE - Poly VVX 450 sound quality issues

Hey All...wondering if any of you have had sound quality complaints using the Poly VVX450 Phones? I have users complaining that calls are of poor sound quality and they frequently have to call callers back...

I have checked the MOS scores for calls and nothing glaring there...I have a VVX myself but have not experienced the issues first hand.

Do any of you have recommendations for other phones? or suggestions with the VVX's