It is worth noting....
My first generation phone is not experiencing any problems or degradation of the voice signal. In fact, there's been no change in quality of voice calls. No apparent change in any other aspect of performance, either. I would venture to suggest that this may be the case with the majority of other users out there as well.
That being the case, before winding back to the old firmware, try restoring the 2.0 firmware again. With your phone plugged in and in iTunes, choose the Restore option. That should blank and restore your iPhone software and all its data. If for some reason there's a glitch in your phone now, that should eradicate it and give you the same solid quality that millions of other users are experiencing.
Trilo