Switching Providers on Iphone 4

Hi gang,

I have been out of the scene for quite some time, and need some help in deciding whether I should change Mobile providers, which will depend on whether my iPhone 4 is unlockable.

I have had a look at the sticky thread " iOS5 Device Overview - Is My Device Jailbreakable/Unlockable?" but it seems to be out of date (I have a jailbroken iPad 1 on 5.1.1 when that thread says it is not jailbreakable) so I thought I better ask first before switching.

I have a white iPhone 4 on 5.1.1(9B208), Model MC606X, Baseband 04.12.01, Serial Number 8612*****00.
It is currently Jailbroken, and I have NOT preserved the baseband with each update as I did not need an unlock.

My internet ISP is offering a much better plan, and I would like to switch it, but I obviously can't if I am not able to unlock my mobile.
Also, I am unable to have the phone unlocked by the provider, as the phone was purchased second hand, and I do not know the original account details.

Any help you guys can share would really be appreciated.
Many thanks in advance.
Kind Regards
Kevin :D

Hi!
You cannot unlock this baseband, unfortunately. Ultrasn0w or SAM won't help you any longer.