Unlocked phones are opposite to contract phones, but do not mean they can work in any phone carriers, as you know, different phone carriers need different network frequencies.
If I buy an unlocked cell phone, and transfer my sim card from my old AT&T Pay as You Go Phone, is it necessary then to go to AT&T Dealer if I want to retain my current number????
Another issue with unlocked cell phones is that they may not work on all GSM bands; as of 2012, a quadband phone could operate anywhere in the world with a , but other phones could not. This may be a concern for international travelers especially, since not all bands are used in all countries. For example, in South America, the 850 MHz band is in common use, but others may not be available. It is advisable to check on which bands the phone uses, and which bands are available in given travel destinations.