Go here:
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[Tutorial] How To Jailbreak Fw 2.1 Using Quickpwn 1.1/Pwnage Tool Error-Free (Mac)
These are the steps on how to jailbreak both iPhone 3g and iPhone classic with original firmware 2.1 without any errors, specifically error 1600.
Steps:
1. Restore your device to the original firmware 2.1.
2. Once restored, you'll see activation mode/screen.
3. Open Quickpwn 1.1(Mac) and follow the steps it tells you, wait until it's done.
4. You'll have now and Activated, jailbroken and unlocked iPhone.
(Optional) :
5: If you want to use your CFW 2.1 created through Pwnage 2.1, simply connect your iPhone to iTunes and press "restore"(no need to DFU, setting to DFU mode is the culprit of error 1600) then locate the CFW 2.1. Wait 'til its done. Enjoy your jailbroken devices!