Metro Atlanta is 404, the outer edges are 770, BUT 678 is a relatively new (seven or so years if I recall correctly) area code that also handles the sprawl areas as the 770 numbers were running out.
Atlanta now uses “overlay” area codes. Anywhere in the metro area could be area code 404, 678, or 770. For instance - my office phone and home phone are 770, our satellite office and my business cell phone are 678, my parents cell phones are 404. All are physically located within 10 miles of each other in the eastern suburbs. A Marietta number (northern suburbs) could well be 678.