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    An ESN is a numeric identifier that uniquely identifies a CDMA phone in the United States. The ESN can typically be found written underneath a phone's battery and is generally written in both decimal and hexadecimal versions. The ESN is what a CDMA network uses to identify a phone and determine which subscriber's account, if any, it is linked to. Because of this, when switching from one phone to another, subscribers will have to provide the ESN of the new phone to the network carrier before it can be activated. This can often be done at the carrier's website. The ESN is slowly being phased out in favor of the MEID, a longer number that is compatible with the IMEI system used in GSM and UMTS phones. This is happening because phone manufacturers are literally running out of ESN identifiers.

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