How can I make a Nokia phone that was on Cellular One system work on ATT system using ATT sim card.?

I have a Nokia 6085 cell phone that was on Cellular One system , Cell One in my area is now ATT. I put my ATT sim card in this phone and have phone and text, However I can't do multimedia and internet both of which are on my plan. Is there a code to unlock these features on this phone? ATT says no.

If the phone is allowing you to make and receive calls and text messages then it is unlocked, you most likely just need to configure it to use AT&T network for MMS and internet. For MMS if you go into messages then use the menu key you should be able to access the MMS settings menu.