There is another way to unlock phone. But you will need SIM from other carrier.
1. Turn off phone
2. Remove ATT SIM & put SIM from other carrier
3. Turn on phone
4. Network lock message will be displayed with option to enter unlock phone
5. Select unlock option & enter unlock code
6. If successful, NO message is displayed & returns to standby screen
You can verify network unlock by code from
http://www.samsung-mythic.com/samsung-mythic-tips-and-tricks/samsung-secret-code%27s/*#7465625# = Check the phone lock status