Palisades Collection - First National Collection Bureau, Inc. AT&T Sparks, Nevada

I recieved a letter in the mail stating that I owe $331.63 for an outstanding collection from the creditor: Palisades Collection, LLC., (AT&T).   I have not had an account with AT&T for over 3 years.  The account was paid in full, never did go into collections.  I tried to call the 1-800-824-6191 phone number which was listed on the bill, and when the call transferred, it disconnected automatically.  I am sending this company a letter today telling them to refrain from sending me bogus bills.  I would like to add that I pulled my credit report from all 3 major credit agencies, and there is NOT anything at all listed for AT&T.  I am wondering if someone has stolen my old AT&T personal information is out there, somewhere using my info for a cell phone.  Any advice would be appreciated.