Hi, I'm Nathan.Sorry to hear that you're having trouble with this but I'll do my best to help you.
Can you tell me a bit more about the problem please:1) If you have multiple handsets (within the same system), does this happen with all of them? 2) Are there other phones or even a fax, outside of this system but on the same line? If so are they working properly?3) Is this a landline from the local phone/cable company, or a VOIP service (vonage/magicjack/etc)?4) Do you have dsl service on the same line? 5) Full model of the phone(s) (printed on sticker on the base, not the handset)
That message is not actually from the panasonic system at all,
if the phone is ringing and giving you this message it is either coming from ATT (unlikely) or is some type of scam trying to get access to your phone/account.
Are you certain you only have a "landline" connection, and perhaps don't have uverse fiber optic service?