I had this useful information from AT&T Customer Service today. A very practical approach of saving money using your iPhone overseas.
• Turn Data Roaming Off: Be sure to download and install the latest version of iPhone software from iTunes. By default, the setting for international data roaming will be in the "Off" position.
To turn data roaming “On/Off” tap on Settings - General - Network - Data Roaming.
• Use Wi-Fi Instead of 3G/GPRS/EDGE: Wi-Fi is available in many international airports, hotels, and restaurants to browse the Web or check e-mail.
• Turn Fetch New Data Off: Check e-mail and sync contacts and calendars manually instead of having the data pushed to your iPhone automatically. This way you can control the flow of data coming to your iPhone.
To turn off the Auto-Check functionality tap on Settings - Mail, Contacts, Calendars - Fetch New Data. Change Push to “Off” and select Fetch Manually.
• Reset the Usage Tracker to Zero: When you arrive overseas, access the usage tracker in the general settings menu and select reset statistics. This will enable you to track your estimated data usage.
To reset Usage Tracker to zero, tap on Settings - General - Usage - Reset.
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