This article describes how to troubleshoot roaming problems.
INSTRUCTIONS & INFO
In order to use your device outside of the United States you must add international roaming to your account. Note: To learn more, visit the AT&T International Plans website.
To troubleshoot issues with roaming, from the home screen tap Settings.
Tap Mobile Broadband.
Tap Int'l Roaming to adjust settings.
When all else fails, conduct a Soft Reset. Press and hold the Power key.