Seriously, sometimes it's just AT&T's network having issues, especially if you hangup and try again, and it works just fine. This happens to me quite a lot.
Probably a CDMA (Verizon) iPhone 4.
Step 14 is missing: insert small spudger or screwdriver on left and right sides to separate the display panel from the metal backing plate.
Quote from loganm:
does this void the warranty?