An electronic serial number, or ESN, is assigned to mobile devices to ensure that the phone is valid for making calls with a carrier's cellular service, notes Techopedia. This number has 32 bits and includes portions for a serial number and the manufacturer code, as well as extra bits that aren't yet used.
Code Division Multiple Access, or CDMA, rather than Global System for Mobile Communications, or GSM, phones usually have ESNs. ESN assignment was originally the United States Communications Commission's idea for fraud prevention. A user who needs to know a device's ESN may find it under the phone's battery or on the phone's packaging, and there's usually a way to get the number from the device's settings, notes Wirefly.