I've had call quality issues of my own (BLUETOOTH).Normal calls are great,but,calls on B/T are much as you described w/your phone.
I've performed many resets since day one,most recently since the latest update from T-MOBILE & still no difference.I had some success w/a JABRA headset,but,that was short-lived & performed poorly soon after w/the ONE,as numerous other B/T headsets have.
I've even exchanged the suspected offending B/T (MOTOROLA ELITE SLIVER),& still no difference,yet,the B/T works fine on other phones.
I'm suspecting your issues are most likely hardware-related,w/a Factory Reset as the last attempt before returning the phone to HTC for warranty service.
I finally did just that w/mine today,as I use my B/T extensively & have exhausted all other solutions.