The SIM card in your Wireless Home Phone device may be missing or not installed properly.
Remove the battery cover and the battery. Make sure a SIM card is inserted, pushed all the way into the SIM card slot and is secure. In some cases, it may be necessary to completely remove and re-insert the SIM card for proper functioning.
If this does not resolve the issue, your SIM card may be invalid or damaged and needs to be replaced. You will need to take your Wireless Home Phone device to the nearest AT&T Store for a replacement SIM card.