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I thought the IMSI unlock saga is done and dusted but apparently there are more to it. All credit goes to Loktar_Sun from weiphone.com who pursued an earlier lead that many, yours truly included, have gave up as a viable exploit.
What do you need: A jailbroken iPhone that can be activated in iTunes (that is, not officially blacklisted), a computer with the latest iTunes installed and working internet connection. You will also need to know the carrier that your iPhone is locked to.
1. Step one: Install Sam Bingner's SAM (Subscriber Artificial Module) package. I recommend that you get it from repo.bingner.com since older versions have a different interface and may not work for this purpose.
2. Enter SAM by either going through the settings menu or find the SAMPrefs icon on your springboard. You will need to have the SIM card you intend to use in your phone.
3. Go to utilities and select "De-Activate iPhone", your ActivationState under "More Information" should now be "Unactivated"
4. With SAM enabled, choose "By Country and Carrier" in "Method"; find your carrier, for some carriers operating more than one Carrier ID you may need to select "SIM ID"; easy to tell since iTunes will not activate if the wrong IMSI is selected.
5. Go to More Information", copy or write down the IMSI in "SAM Details", then tap "Spoof Real SIM to SAM".
6. Go back to the main SAM screen and change your "Method" to manual. Paste or enter the IMSI string we saved in Step 5.
7. Connect your iPhone to your computer and allow iTunes to do its job (namely re-activating your phone), double click "Phone Number" parameter at the main device screen and make sure that the ICCID matches that of your SIM card. If not you need to start over from Step 1.
8. Unplug your phone, close iTunes.
9. Disable SAM. The source article says to uninstall SAM and delete your lockdownd folders; it's unnecessary.10. Connect your phone to iTunes again, you should get an error saying that your phone cannot be activated. This is normal. Just close iTunes and open it again.
11. You should see signal bars in a short time, congratulations.
12. Push notifications may stop working after this procedure but can be easily restored with "clear push" utility in SAM followed by connecting to iTunes.
Your phone will work normally with your SIM card and that SIM card ONLY since we have tricked iTunes to think that ICCID is one of the intended carrier's. The phone can be rebooted and connected to iTunes freely without losing the "unlock", at least for now.
Since it does not involve emergency numbers or IMSI spoofing it will not have any of the issues associated with SIM interposers. Tested working on iOS 5.01 but should work with tethered 5.1 too.
