This is not correct. Please check your facts before posting.

The real problem is that the device is from the original AT&T Wireless, and the SIM card you have is from the new at&t Mobility, previously known as Cingular Wireless. AT&T Wireless phones cannot accept Cingular or at&t SIM cards. As such, you will need to get the device unlocked to use a SIM card from the current at&t.