Samsung SGH-P207

Hi I am on a family plan with my parents. We happened to activate the family plan about two months before att was taken over by cingular. I also got my phone after our plan was activated. The problem is that we got a plan activated in a satellite store like cell tech or something like that. SO basically I have an att sim card with a cingular phone (z500) that i do not like and want to know if i can trade my phone in through cingular and if i can, how much credit will i recieve on my perfect condition z500. I want to trade in my z500 for this but i dont even know if it is in stores or unlocked. Info please 😳