When a Cell phone is purchased from a service provider, it is locked in such a way that only that provider will be accepted and utilized by the phone. When it is “unlocked”, any provider that uses that network can now use that phone. There is a difference though because some carriers use GSM and some use CDMA.

Verizon, Sprint, Nextel, Cricket and Alltel all operate using CDMA networks, so if the “unlocked” phone was originally on one of those service providers, it will generally only work with them. There are phone exceptions to this though.

Cingular, AT&T, T-Mobile, Rogers Wireless and many other Global GSM Provider worldwide operate on a GSM network, so same as above.

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Unlocked Cellular Phone is a post from: Nokia Stuff