If you have updated your iPhone 3G(S) baseband to iPad baseband 06.15.00 in order to unlock it on iOS 4.2.1 and now you are having issues while restoring the device, Sherif Hashif has posted a simple method which we are going to share with you people.
For PwnageTool users:
A : If you are on Mac and use PwnageTool: If you used pwnage tool then you will have to create a “new” custom 4.1 or 4.2.1 and say “No” when you are offered the BB update to 6.15.00 since you already have it, then build the custom ipsw and restore to that.For Redsn0w users:
B : If you use(d) Redsn0w:How to go again to iOS 4.2.1:
B.1: If you are currently on 4.1, then you can restore to official 4.1 again, and when you see error 1015 pop-up, you kick your device out of recovery mode using TinyUmbrella, use redsn0w and don’t check “ipad BB update” because you already have that Here Is How To Restore iPhone 3G And iPhone 3GS While On Baseband 06.15.00 And Having Issues icon smile
B.2: If you are currently on 4.2.1 then you’ll have to restore back to 4.1 (because 4.2.1 will not allow you to kick the phone out of recovery when 1015 occurs due to the silly BB / iOS mismatch check added there) , then do the same steps as CASE-1.
If you want to go again to 4.2.1 your only way is to use a custom 4.2.1 done by pwnage tool in the same steps mentioned in A .Try Hashim's methods above to restore you device between 4.2.1 and 4.1, it worth trying. Don't forget to tell us about your experience.