I finally got it worked. First, the 8 digits number is wrong. I just called AT&T again. The guy ask my IMEI this time and gave me 16 digits unlock code. Second, when I tried the 8 digits number, the number of the "Network" changed from 5 to 4. However, after I used the 16 digits, it not only unlock the phone but also change the number of the "Network" back to 5. Don't understand but I think it's good.:D
I think anyone can try this way. If you or your friends are AT&T customer for more than 6 months, you can, or ask your friends to, call AT&T to get the unlock code. Hope this can help.:)




