If you are unable to connect to the AT&T network after activating your iPhone 5 via iTunes (e.g. No Network is displayed on top of your phone), here are some tips that could fix the problem:
- Manually activate your new phone via the AT&T website https://www.wireless.att.com/activations/.
- Verify that the IMEI number listed matches your new iPhone 5. You can check this by going to Settings on your iPhone 5, then selecting General, and About.
- After activating the phone via the website, wait around 5 minutes and restart your iPhone 5.
- If this does not work, call the AT&T activation number at: 18003310500. They should be able to help with the activation issue.
Source: http://forums.att.com/t5/Apple-Community-Discussion/iPhone-5-not-activating/td-p/3307081
