AT&T to Cingular - Sim cards????

So, I have my old V600 on AT&T network and I want to give this phone to my daughter to use since I got a new phone. The Cingular folks are telling me that I cant do that because she is currently on Cingular and the old At&T phone won't accept the new Cingular SIM cards.

That sounds werid to me. Can anyone help me out? Her old phone was stolen, so I was looking into not having to buy her a new phone, but give her my old one.

Thanks!!!

It will work with a Cingular sim card, but you'll need to get it unlocked first. Local retail stores sometimes carry the unlocking equipment, or you can search online for it. Google something like "Unlock Motorola V600."

Thanks!