In order to use your AT&T device abroad, you’ll need to either purchase an international roaming add-on package or activate roaming on your wireless device.
Once you’ve activated an international add-on or enabled international roaming, your phone will immediately attempt to located the nearest wireless network. Once a network is located in the top left or right hand corner of your device, you should have the ability to make and receive calls.
How to call United States from UK:
00 is the United Kingdom code to dial first when calling internationally. The US country code must be dialed next, i.e. > 00 + 1 + Area Code + Local Number.
To call or send a text to another country, dial +, followed by the country code and local number.