There are a few different ways that mobile carriers identify handsets on their network. The International Mobile Equipment Identity or IMEI code is unique to GSM (Global System for Mobile communications) networks. The largest GSM networks in the United States are AT&T Wireless and T-Mobile. Electronic Serial Numbers (ESN) are used on Code Division Multiple Access (CDMA) networks. The largest CDMA networks in the United States are Sprint and Verizon Wireless.
What is the purpose of an IMEI number?
An IMEI number is a 15 to 17 digit number unique to each cell phone registered on a GSM network. The number uniquely identifies a handset on each network. A phone is only given one IMEI number. It can be used in the case of when a phone is reported stolen. In a situation like that, the user can call the carrier and have the IMEI banned. This will render the phone unusable and will block the phone from being used on the network. GSM phones are equipped with SIM cards that are unique to the user and not the phone. If an IMEI is banned and the cell phone is not working on the network, putting a new SIM card in the phone will not make the phone work.
What is the purpose of an ESN?
ESN's are also unique identifiers for cellphones. The manufacturer of the handset, not the mobile carrier, places them on the phones. It is a 32-digit number that is programmed into the phone and also printed on a small sticker under the batteries of handsets. The ESN is linked to the user's account and holds his or her information including network and account information.
ESN as SIM card and IMEI number
The main difference between an ESN and an IMEI is that an ESN serves as both the IMEI number and the SIM card because it carries both the network (user) information and the cell phone's unique identifier. However, because this number serves as both, if a user decides to change phones, it is not as simple as switching out a SIM card. The carrier must be informed of the switch, which the customer can do online and over the telephone.
According to the Telecommunications Industry Association website, as of 2004 they have started to phase out ESN numbers in favor of MEID (Mobile Equipment ID) numbers, which are essentially the same thing, but are 56 characters long instead of 32. Additionally, ESN's can use hexadecimal digits. IMEI numbers can only be numbers, decimal digits only. ESN's can be alphabetic or numerical, called hexadecimal, digits.