Unlocking a Motorola V557

I bought a V557 on ebay in a hurry becoz the ad said it was a quad band world phone. Like an idiot, I assumed it was already unlocked. When I got the phone in the mail, it was a Cingular badged phone. In any case, I turned it on and it said unregistered SIM. I naturally thought that it was unlocked. However, I tried a T-Mobile sim in it and it is asking for some code or other. I need to use this phone overseas and I was wondering how to unlock it. I heard that it can be unlocked using a computer and the USB data cable. Can someone please help on this?
Many thanks.

you can have it remotely unlocked or you can go to an independent cell phone shop, they usually unlock phones as well.