I'm an AT&T premises tech. I specialize in the installation of all U-Verse services in the home including whole home DVR, TV services and Internet. I Educate customers on the service functions and features.
It means your SIM card isn't activated in that phone or it's damaged. If you're trying to switch phones by switching SIM cards it may not work with all phones as all SIM cards are not compatible with all phones.
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Is this an att sim in an att phone? If not an att sim, your device will have to be unlocked by calling 611 or 800-331--0500. Otherwise the sim is damaged or not active. Try the sim in another att device to see if it works, if not the sim is bad. If it does work in another device your device is bad. Hope this helps!
Your service has been shut off or suspended (meaning reported lost/stolen)
you need to call at&t and tell them that your SIM card is not registered with your service. Take the SIM card out of the phone give them the long number on the SIM card. Your service and phone number may be active but they have not activated the SIM card to work with your phone. There will be no fee to take care of this and it takes a few seconds once they put in the SIM card number