I don't think its a "network" issue, "all the time". From what I understand, based on CDMA technology, and some of the Droid's more advanced settings, as signal strength/bandwidth decreases, compression increases, lessening audio quality. Factor in whatever hadware there is in the phone itself, that make up whatever sound quality one may experience (not all phones are equal, esp. considering reception:
http://androidforums.com/motorola-d...rough-3g-1x-switching.html?highlight=homemade)
Given the flaky reception I had with "both" of my Droids, I can see how call quality can vary greatly, especially if there is any noise cancelling is involved, and not perfected yet...it is a new platform.
My first call on the Droid was to voice mail, in a low signal area, and it sounded really compressed and slightly distorted...as if I was calling the other side of the world with a calling card service. Other local calls, to land lines sounded ok, as well as other cell phones. I didn't have a lot of time (2 weeks) to gauge/monitor/test every call type, over any extended period of time, but if nothing else, the Droid on Verizon is "capable" of good call quality, in both directions.
FWIW, colnago = VoIP engineer, and self-proclaimed audiophile, with full conrad-johnson tube audio system.