Well,i think there is some setting for the internet and few other things,but voice work without changing any setting!!!

That is true I lived in Dubai and actually used the sim cards from over there in my iphone.There is some settings for the internet and some other things but voice should work without changing any settings. You do need to have it unlocked for this to work overseas.

That's a new one... Would you be moving to and securing new service in this other country, or just traveling? I have a 3GS on AT&T, and I live in the USA. I travel to Canada fairly often in the summer, for a couple of days to a week or two at a time. All I have to do to use my iPhone in Canada is have AT&T put an international plan on my contract for the time I'll be there. That's what the rep told me, anyway. And that's just to avoid paying international roaming charges, which can add up in a real quick hurry.

I am planning to get an iPhone 3GS, but then my friends told me that if i get it in one country and to use it in another country, i have to change the codes (i dnt even know what codes they're talking about) - is that true ?