IPhone - Downgrade to an earlier version / 3G with 3.1.3 and 05.12.01

I have it Jailbroken fine, but I now need to unlock it to use it on another network. It appears that there is not a way to do that now. Is there a way to 'downgrade' the phone to an earlier version.

Solution:

Please click following links for solution

http://www.howardforums.com/showthread.php/1632796-3G-with-3.1.3-and-05.12.01-what-are-my-options

http://modmyi.com/forums/unlocking-activation/703164-3g-iphone-os-3-1-3-baseband-05-12-01-bootloader-05-09-possible-unlock.html