If you have purchased an iPhone but do not have a contract with AT&T, you will notice that when you plug your iPhone into iTunes you cannot activate it. The team from ModMyiPhone has released a program that you can use to activate it for use on your GSM network. The program is simple and can be used on Mac computers.
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Download and install the iActivator program onto your Mac computer.
Plug your iPhone into your Mac and wait for it to be recognized. If you have iTunes installed, which you more than likely do because it comes with all Mac computers out of the box, it may open, and you must exit the program.
Open iActivator and you will be see a graphical user interface where you can press "Generate Activation" and wait as the software creates a file that will be used to activate the phone. When it is finished, press "Activate iPhone" and use the file the program just created, and your phone will be activated without using iTunes.
Unplug your iPhone from your Mac and insert your sim card into your iPhone sim card slot. You will now be able to use your iPhone as not only an iPod, but as a phone.