You can use iPhone to make calls in many countries around the world. You must first enable your carrier’s service plan for international roaming.

Enable international roaming:
Contact your carrier.
Set iPhone to add the correct prefix when dialing from another country:
In Settings, tap Phone, then turn International Assist on. This lets you make calls to your home country using the numbers in your contacts and favorites, without having to add a prefix or your country code.
Set the carrier to use:
In Settings, tap Carrier, then select the carrier you prefer. This option is available only when you’re traveling outside your carrier’s network. You can make calls only on carriers that have roaming agreements with your iPhone service provider.

IMPORTANT: Voice and data roaming charges may apply. To avoid data roaming charges, turn Data Roaming off.

Turn Data Roaming on or off:
In Settings, choose General > Network and turn Data Roaming on or off. Data Roaming is turned off by default.

Turning Data Roaming off helps to avoid data roaming charges when traveling outside your carrier’s network by disabling data transmission over the cellular network. You can still access the Internet if you have a Wi-Fi connection. If Wi-Fi network access is not available, however, you cannot:

  • Get visual voicemail
  • Send or receive email
  • Browse the Internet
  • Sync your contacts, calendars, or bookmarks with MobileMe or Exchange
  • Send or receive text messages
  • Stream YouTube videos
  • Get stock quotes
  • Get maps locations
  • Get weather reports
  • Purchase music or applications

If Data Roaming is turned off, you can still make and receive phone calls, and send and receive text messages. Voice roaming charges may apply. Visual voicemail is delivered if there is no charge; if your carrier charges for delivery of visual voicemail when roaming, turning Data Roaming off prevents the delivery of visual voicemail until you turn Data Roaming back on or return to an area where your carrier’s network is available.

To enable email, web browsing, and other data services, turn Data Roaming on.

IMPORTANT: If Data Roaming is turned on, you may incur charges when roaming outside your carrier’s network for the use of any of the features listed above, as well as for delivery of visual voicemail. Check with your carrier for information about roaming charges.

Get voicemail when visual voicemail isn’t available:
Dial your own number, or press and hold “1” on the keypad.
Avoid roaming charges by using Airplane Mode:
In Settings, tap Airplane Mode to turn it on, then tap Wi-Fi and turn Wi-Fi on.
http://www.apple.com/iphone/tips/