Can I unlock an Alltel Blackberry and use it for AT&T?

My dad is giving me an extra blackberry curve from Alltel that he isn't using. I'm not sure which model it is, but he got it within the last year so I assume it is one of the models they still carry. I currently have AT&T, and I've seen mixed answers on if I can use it or not. Is there anything I can do to get it to work? Or am I out of luck on this?
Just got it and its the blackberry curve 8530. Don't really know if that makes a difference? Alltel sent him this phone when he had trouble with his other phone over a year ago, but I guess he just never sent it back. It's what they called an XBM replacement phone. Anyone know if this makes a difference also? Thanks!