Originally Posted by Palal

It's generally easier to reprogram all your numbers in your phone (including the US numbers) to include a + before the country code (1 for the US). This way you can dial any number in your phone from anywhere in the world.

All my phone numbers are now:
+[country code][phone number]
So, for the US it's :
+14151234567
For, say, Czech Republic it would be
+420[phone]
Germany:
+49[area code+phone]

Makes things very easy without having to remember local prefixes for dialing international or local numbers. GSM is smart enough to determine what to dial.