After being Jailbroken/Unlocked still can't use go phone sim

[jlegend27]

My 16gb Iphone 3g was updated to 4.2.1 and after jailbraking and unlocking the phone i still can't use my AT&T go phone sim card inside of it. I tried downgrading my iOS back to 4.0 but apple wouldn't give me the permission when i hit restore. And now i'm wondering if anyone can help me?

[i.Annie]

What's the modem firmware version. If you upgraded to a stock 4.2.1, you will have the updated baseband, which is not unlockable.

If you want to unlock and accept the risks of the new unlock for 4.2.1, such as possible GPS loss and a void of warranty (if applicable), then proceed to do so at your own risk.

You may unlock by updating the baseband to the iPad baseband of 6.15.00. If you can confirm that the baseband is not 5.13.04 and below, then you will know whether you need to update the baseband or not.

You may also check in Cydia with Fuzzyband to see if your bootloader is 5.8 and can downgrade your baseband rather than update. It is always better to downgrade than upgrade the baseband.