If you own a phone that operates on the GSM network, then the device contains a SIM card that contains your personal data and your phone's identifying information. SIM cloning was originally developed because people wanted to be able to create a backup copy of their phone. However, due to fraud and safety concerns, use of a cloned SIM is illegal in many countries including the United States.
SIM cloning is the process by which you copy the SIM card in a phone operating on the GSM communication system. The SIM card contains all of the phone's data, so copying a SIM card is akin to copying your address book, messages and other personal data on to a new card. More importantly, the clone card also contains the same identifying characteristics as your original card.
The primary purpose of engaging in SIM cloning is so that you can use two different cell phones with one cell number and account. Any calls that are made to the phone will ring on both the original phone and the clone. All calls that are made from the clones and all charges that are incurred using the clone appear on the same bill as the original phone.
There are two primary steps to perform a SIM card clone. The first step is to locate the phone's identifying information. The SIM card is placed into a special reader, and then the reader extracts the phone's identifying IMSI number and authentication "ki." The program can optionally copy all data on the card as well. Once the appropriate data is located then a writing device is used to modify another SIM card to take on the same IMSI, ki and contents as your original card.
Creating and using a SIM card clone is illegal in both the United States and the United Kingdom. This prohibition is partially for fraud prevention reasons, since a criminal can copy a phone and then charge it to another person. The prohibition is also for safety reasons since emergency services may be trace the location of an emergency call made using a SIM clone to the wrong geographic point. If you live outside the US and UK then you should learn about the legality of SIM cloning in your area before considering cloning your phone.