The Electronic Serial Number (ESN) is the unique identification number embedded or inscribed on the microchip in a cellular phone by the manufacturer.
Each time a call is placed, the ESN is automatically transmitted to the base station so the carrier's mobile switching office can check the call's validity.
The ESN cannot easily be altered in the field. The ESN differs from the mobile identification number, which is the wireless carrier's identifier for a phone in the network. MINs and ESNs can be electronically checked to help prevent fraud.