Dial ##778 (pst) on your phone
click edit
if it asks for a code, you will have to figure it out through your carrier. verizon's key is 000000
might work for others.
at the top left side you will see a drop down menu.
Drop it down and select "CDMA Settings" menu. Change "EVRC Enabled" to "Disable" and change the three values underneath it ("Home Page", "Home Orig", "Roam Orig") to "Voice 13K".
this actually ups the voice quality which in turn seems to up the in call volume as well.
Hope this helps