To change a cell phone's International Mobile Station Equipment Identity number, a new IMEI must be flashed to the device. The phone is connected to a computer via a device called a flasher. Flashers allow software running on the computer to change the phone's IMEI number, causing the cell phone carrier to identify it as a different device.
Changing a phone's IMEI number is illegal in some jurisdictions, even when done by the device's legal owner. These laws exist because flashing a new IMEI number is a crucial step in selling a stolen phone. The new number allows the stolen device to be used on networks that blacklist IMEI numbers based on theft reports. Because the IMEI number is used only by the carrier to identify the device on its network, there is no reason for a consumer who has legitimately purchased a phone to change this identifier.
Flashing a phone's IMEI number is legal in the United States as of 2014, but legislation to ban the practice has been introduced in the Senate. Under this proposed law, tampering with the IMEI number of a phone carries a sentence of up to five years in federal prison. This statute criminalizes changing the number regardless of the ownership status of the device.