Easy because CDMA phones can work on ANY CDMA Network (ie: VZW, Sprint, Cricket/Revol/MetroPCS) although CDMA carriers do NOT allow you to bring a phone to them. Even though ANY CDMA phone WILL work. But these carriers do not want this to happen.
iPhone is different due to its GSM, and if you jail break it, it will work on T-Mobile or any other GSM carrier.
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