Sim Not compatible Error Iphone 5

I have a GSM Iphone 5 that is not Icloud locked. IMEI ****

I can't activate it no matter what sim I put in it says sim not compatible.

I have checked the ICCID that comes up on the info screen and it says it is AT&T USA. But an AT&T SIM won't allow me to activate the phone.

I have been to the AT&T web site and tried to Unlock it. Response back was it is not in their Network.

Any ideas?

<Personal Information Edited by Host>