Locked to carrier - not in all countries

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I was thinking that the iPhone could only be used with the carrier from whom you buy it, and you have to unlock and(or) jailbreak to use it with another carrier. But it turns out that iPhones are LOCKED to CARRIER in only a few countries. In countries like Singapore, you buy an iPhone from SingTel and take away the SIM, throw in any SIM you like and use it! I just want to know if its true! Can some one confirm me? The Apple website link is here: http://support.apple.com/kb/HT1937