Haven't seen this mentioned in the X-specific forums yet, and not sure how many of you wander outside the X forums so I thought I'd share this. You can potentially improve your voice quality by toggling a hidden setting by going to your dialer and dialing ##PROGRAM (##7764726) and hitting Send. The SPC password is 000000 on the DX. If you select option 4 (Vocoder) you will be given 3 options for your voice codec used during voice calls. The default on the DX is EVRC. The other options are the old 13K codec which is a constant bitrate codec, which some think is more crisp but has the potential for more background noise/hiss, and the new EVRC-B. EVRC-B will give you better voice quality at the same bitrate as EVRC.
Enhanced Variable Rate Codec B - Wikipedia
After changing codecs, your phone will reboot to start using the new codec. Your mileage may vary, but my voice quality was definitely improved switching to EVRC-B. Also note that this is sanctioned by Verizon, but they prefer you use EVRC or EVRC-B when you can because it uses less bandwidth on their towers. They only suggest 13K as a last resort fix for voice call problems.