If you have a GSM phone and it is unlocked (i.e. not locked to any specific carrier) then all you do is transfer the SIM card to that phone and it works. No need to register the phone with the carrier.
If it is a tri- or quad band phone and it is "unlocked" it can be used in almost any country around the world.
e.g. if you travel to UK, you can buy a local SIM card either vrom Vodafone or Virgin Mobile or Orange, etc. and just insert thye SIM into your phone and it will work. You can call USA from there but you get charged for minutes used. Usually more expensive than a local call. Once you are low in your minutes, you add more minutes, either by calling the carrier and giving them a credit card number or by buying a card with minutes and adding to your existing card via your cell phone. Is that clear?
i've been lookin to get a phone that just about has it all, but livin here in the states thats pretty impossible. Especially since by the time all those nice cell phones from across the seas get here, they already have ones 5 times better. Anyway, my question is can i take a random phone not out in the states yet, like a Sony ericsson w800i or a Nokia N91 which are both tri-bands, and have them activated by any gsm provider in North America? by the way people ask SIM ?s and the answers they recieve, it sounds that way. But i also hear alot talk about the providers not activating phones because it wasnt a phone they provide. And another ?, so i get a a tri- or quad- band phone, fly over to europe, and what? buy a sim from whatever country i'm in and it will come with as many minutes that i buy? and if so, i can use those minutes to call the U.S. at no extra charge?