Saw this tip over at HoFo:
Get into the programming menu, from the call screen, enter ##PROGRAM and press send. Enter code 000000.
In option 04, you can change the vocode option to EVRC-B, it made my in-call audio sound better. Your phone will reset after you exit and apply the new setting.
Warning: Be careful what options you change in the programming menu, you could potentially brick your phone.