3G Unlock Not Working on 2.2.1/2.28.00

[Mannyak]

Normally I use my iPhone 3G on AT&T and it's fine, but I still didn't upgrade the baseband just in case I wanted to unlock. Now I traveled out of the country for vacation and want to use a different sim card, so I installed yellowsn0w through Cydia. But when I try to use the new sim card it tells me "SIM Failure".

Anyone know why?

 

[Mannyak]

I know it's probably something stupid, but I really need help on this, I'm here for a month and phones here are pretty expensive.

So bump.