Will an at&t SIM card work in an old cingular phone?

I just got a new phone on at&t and I'll be going to the beach soon.. I don't want to ruin the phone so I was going to try and put my SIM card in an old phone.. but the only other phone I have is made for Cingular. The old phone is the Sony Ericsson z300a. If I put my SIM card in this phone will it still work the same? Please help me out. :)

Do you want to use your Sony Ericsson Z300A with AT&T USA network sim card?You need to unlock the phone from current Cingular network.Otherwise it will display "Sim Not Valid" or "Invalid Sim card" message on screen.To fix this problem get unlock code from the site Prounlocking.com for Sony Ericsson Z300A and unlock it easily.I have unlocked my Sony Ericsson Z300A from same cingular USA using the unlock code from the site.