Originally Posted by DEVIS
What you're talking about is called SIM Cloning and it is highly illegal. It can be done, with certain underground equipment
It is NOT illegal everywhere, only certain countries. And the so called "underground equipment" is available everywhere from Ebay to many local phone stores.
Also, calling it "highly illegal" seems a little OTT; killing someone is highly illegal, but copying a sim card to service you pay for every month is just regular illegal, and unless you are doing this 1000 times a week as a commercial service, there is very little chance you'll everhear anything about it again.