Previously on the iPhone 4 it was possible to use TinyUmbrella to prevent baseband upgrade when restoring or upgrading iOS 4.x

The tinyumbrella feature still prevents baseband update but Apple has thrown another barrier in the way.

Apple have included an extra requirement in the restore logic for iOS 4.2.1 that will prevent the restore from completing if the iPhone baseband is not 3.10.01, this cannot be bypassed with the "exit recovery mode" type solutions previously used

The two solutions are:

1. downgrade again to iOS 4.1 or lower (so your baseband doesn't get upgraded).
2. Wait for an updated pwnagetool that can disable this extra check.

The current version of ultrasn0w doesn't (yet) unlock 1.59.00 on iOS 4.2.1 anyway, so if you rely on unlock, stay with your currently stable iOS 4 version.

With this extra requirement, going forward the only viable solution to prevent baseband update is PwnageTool. Apple cannot block pwnagetool so easily.

i accidentally upgraded to 4.2 and my base band also got upgraded to 03.10.01. NOW my phone shows a message "No sim card installed" and i can only make emergency calls ....
Please help (Forget about the jailbreak at least get me out of the "no sim card installed screen" )

atleast make my iphone work as itouch

You can revert back to previous baseband from ios 4.1 while apple is still signing it.
Download the stock 4.1 firmware and restore it to your iphone4 NOW. Make sure the hosts file is not redirecting apple.gs.com to localhost or to Cydia.
Do it while you still have time

downgraded iphone4 to 4.1 from 4.2 ... now the baseband is 02.10.04
But i am still stuck on the emergency calling screen .it says insert a valid sim to activate.... but i dont have the original sim (actually the phone was used by my brother in UK before sending it to me in India here).

WHAT could be done now?
.
.

Used limera1n and now my iphone 4 is atleast working as an ipod touch ....
NO emergengy calling ... now the only thing is to unlock it ....
thanks all

Please avoid limera1n when you need hacktivation - it has bad hacktivation logic.

Instead use redsn0w 0.9.6beta4 and run jailbreak with the SIM you want to use inserted.

Not at this time to preserve BB on ip4 if you want to go to 4.2.1.
Plus even if you can, 4.2.1 is a tethered jailbreak.
Best is to stay at 4.0 - 4.1 at this time
Current ultrasn0w 1.2 and previous 1.1-1 can unlock 05.19.00.

Originally Posted by varun89yadav

Used limera1n and now my iphone 4 is atleast working as an ipod touch ....
NO emergengy calling ... now the only thing is to unlock it ....
thanks all

Have u unlocked?

Originally Posted by varun89yadav

Used limera1n and now my iphone 4 is atleast working as an ipod touch ....
NO emergengy calling ... now the only thing is to unlock it ....
thanks all

Originally Posted by jokatts

Have u unlocked?

There is no unlock for 03.10.01 or 02.10.04 baseband on the iPhone 4 yet.

It is too late to use Apple's baseband signing system to downgrade to an unlockable baseband. The last time that this was possible was 9 September 2010.

You might still be able to downgrade from baseband 03.10.01 (the one that shipped with 4.2.1) to 02.10.04 (the one that shipped with 4.1). According to SHSH - The iPhone Wiki Apple is still signing 4.1 restores. This would not offer an unlock, but it might give more chances of a future unlock if for example, the iPhone devteam find an exploit that is only useful in 02.10.04

Originally Posted by thebeej

I'm currently on iOS 4.1 with baseband 1.59.00 on my iPhone 4. I require ultrasn0w. Is there anyway to upgrade to to 4.2 without updating the baseband? The new ultrasn0w can unlock 1.59.00

I explained this in the first post, with unsupported and unofficial tools, it is possible but strongly not recommended as you are limited to a tethered jailbreak.

Wait for updated tools and an untethered jailbreak.