Some Droid users were having the same call quality issues and someone suggested this fix. Worked for some, not for others. As always, be careful if applying:
http://androidforums.com/how-tips/18639-read-improve-call-sound-quality.html?highlight=vocode
http://androidforums.com/how-tips/18639-read-improve-call-sound-quality.html?highlight=vocode