Cloning your mobile phone number gives you the opportunity to use the number on more than one phone. This can come in handy when you are traveling and would rather use a cheap phone instead of your more expensive high-end version, in case it gets lost or stolen. Just be aware that cloning any phone that does not belong to you, and that the owner does not know about, is illegal. If you want to clone your own phone, you can do so in a few steps.
Find the secret menu key to unlock your phone. You can get this key through several avenues but one of the more popular sites is cellphonehacks.com (see Resources). There, search for both of your phone models and write down the hack codes.
Enter your secret key on the phone with service, which will take you to a menu that displays your ESN, or electronic serial number. Write this number down and exit out of the screen by hitting end.
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Repeat step 2 for the phone without service to get to the secret menu. Scroll down to the ESN and write it down. Replace it with the ESN from the service phone. To change the number you will have to find the code, which can also be found at place likes www.cellphonehacks.com. For instance, a Nokia number changer is #639#.
Change the number in the clone to match the number in the original phone and you should have two fully functioning cell phones. Store the ESN for the clone in case you need to reverse the process.