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 your iDevice, below is a simple tutorial to restore iPhone 3G(S) while on baseband 06.15.00 and having issues.

In case you want to restore iPhone 3GS back to iOS 4.1, you must have saved SHSH blobs for 4.1 IPSW.

A) If you are on Mac and use PwnageTool : 

If you used PwnageTool then you will have to create a “new” custom iOS 4.1 or iOS 4.2.1 and say “No” when you are offered the Baseband update to 6.15.00 since you already have it, then build the custom ipsw and restore to that.

B) If you use(d) Redsn0w :

Case 1 : If you are currently on iOS 4.1, then you can restore to official iOS 4.1 again, and when you see error 1015 pop-up, kick your device out of recovery mode using TinyUmbrella, use Redsn0w and don’t check “ipad BB update” because you already have it

Case 2 : If you are currently on iOS 4.2.1 then you will have to restore back to 4.1 (because 4.2.1 will not allow you to kick the phone out of recovery when error 1015 occurs due to the Baseband / iOS mismatch check added there) , then do the same steps as in 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.

It is to remind you that the Dev Team is also working on a new hack which will also allow users to downgrade the baseband from 06.15.00 to any previous baseband (like 05.15.00).