It is possible to activate your iPhone with another SIM. Apple currently offers iPhones in the United States exclusively for use on AT&T's network. You can, however, buy an iPhone from an Apple retail store or an AT&T retail store without a service contract. The iPhone will still be locked to the AT&T network. You will need to jailbreak and unlock your iPhone in order to use it with another SIM. By following these steps, you will be able to use your iPhone on any GSM network with another SIM.
Open iTunes. Select iTunes from the menu bar at the top of the screen, then select "About iTunes." Make sure you have iTunes version 9.0 or later. If you have an older version, select, "Check for Updates," and update your iTunes to the current version. Connect your iPhone to the computer using the iPhone data cable. Do not use the docking device.
Download the Blackra1n jailbreak utility for your computer. You can find a link to the Blackra1n jailbreak utility in the resources section of this article. Follow the onscreen prompts to install the utility to your computer's desktop.
Open the Blackra1n jailbreak utility by clicking the Blackra1n icon that was installed on your destop in step No. 2. A box will open. Click the box that says, "make it ra1n." Click "OK". Your iPhone will now be placed in recovery mode. Your iPhone screen will show a picture of a young man surrounded by floating iPhones.
Wait for your iPhone to reboot. This process takes up to five minutes. Once the iPhone reboots, it will be jailbroken and you will see a new Blackra1n icon on the screen of your iPhone. Make sure your iPhone is connected to a Wi-Fi network. Press the Blackra1n icon on your iPhone screen. You will now be given the chance to select packages. Select "Cydia" and "sn0w." Wait until both packages install. This process takes about two minutes. Press "close". Once you complete this process, your iPhone will automatically restart.
Look at the top of your iPhone. To the right of the headphone jack, you will see a small hole. Push a pin into the hole to remove your existing SIM card. Insert another SIM card in the empty slot on the top of your iPhone. Wait two minutes. You should see the logo in the upper left-hand corner of your iPhone's screen from the cellular provider of the new SIM. If you do not see the new logo, restart your iPhone. You have now activated your iPhone with another SIM card and can use your iPhone on the new SIM's cellular network.
Jailbreaking and unlocking your iPhone may effect the terms of your Apple warranty.