You can probably stop at your local att store and get the right sim chip for free. I got a free sim chip when this happened to me from att. If they don't have the right sim chip, then you need to get a new phone.
i have an old Treo handspring 180 phone that i used for a while back in 2003 w/ an ATT service we had. In 2006 we got a new ATT contract/service with new phones and we've had that ever since.
However, a few days ago, the phone i was using (sony ericcson) broke, and i thought about reusing the Treo handspring as i always loved this phone even though it's old, but today i tried to put in my cingular SIM card and it didn't fit, it's too big for the phone. it's a 3G 64K card.
I don't know what to do. i thought all SIM cards were the same. is there anything i can do to be able to use this old phone. thanks all!