What is cell phone cloning?

Cell phone cloning is copying the identity of one mobile telephone to another mobile telephone.
Usually this is done for the purpose of making fraudulent telephone calls. The bill for the calls go to the legitimate subscriber. This made cloning very popular in areas with large immigrant populations, where the cost to "call home" was very steep. The cloner is also able to make effectively anonymous calls, which attracts another group of interested users.
Cell phone cloning started with Motorola "bag" phones and reached it's peak in the mid 90's with a commonly available modification for the Motorola "brick" phones, such as the Classic, the Ultra Classic, and the Model 8000.

Cloning involved modifying or replacing the EPROM in the phone with a new chip which would allow you to configure an ESN (Electronic Serial Number) via software. You would also have to change the MIN (Mobile Identification Number).
When you had successfully changed the ESN/MIN pair, your phone was an effective clone of the other phone.
Cloning required access to ESN and MIN pairs. ESN/MIN pairs were discovered in several ways:
Sniffing the cellular network Trashing cellular companies or cellular resellers Hacking cellular companies or cellular resellers Cloning still works under the AMPS/NAMPS system, but has fallen in popularity as older cloneable phones are more difficult to find and newer phones have not been successfully reverse-engineered.
Cloning has been successfully demonstrated under GSM, but the process is not easy and it currently remains in the realm of serious hobbiests and researchers.