ATT's UPGRADE (downgrade) to its network
https://www.wireless.att.com/support/tutorials.do? »
If you still have problems after that, try Nokia technical support. At first, they will tell you to talk to your provider (AT&T), but if you explain that it is an unlocked phone and AT&T will not help you, they should help you by checking manufacturer settings.
Also, try these:
1. Power the phone on and off (You probably already have, but this solves A LOT of issue...
(continues)