EDIT#2: It does work. I just had to do a master reset , reset settings
EDIT#1: I've tried this but my in the service menu >> system info >> Configuration it still says AMR_FR EFR FR AMR_HR HR . Perhaps this doesn't work for W580i?
What I gather is that AT&T/Cingular doesn't implement AMR very well and so if you debrand, which enables AMR, you might experience poor call quality. I think I have noticed this myself.
I'm going to try this file edit and remove AMR-HR