In my GSM Treo 650 from what it seems the 8khz audio coming from your voice calls are being upsampled loosely by my Treo 650 making it very noticeable and very annoying.
Somewhere in the audio data path of the treo during a call those 8khz are being upsampled to 44.1khz or 48khz without any kind of filtered rate conversion. The result is crappy sounding calls with lots of high frequency noise and other kinds of artifacts.

Was this happening on the Treo 600? Is this happening on the Treo 650?