When the area code system was implemented, back in 1951, the 404 area code was applied to all of Georgia. But there are more codes under that 404 layer. Back in 1954, there was a split and the area code 912 took over middle and southern Georgia. Since August of 2000, the area code 229 covered the southwest and central was covered by area code 478.
People began to complain. The residents of towns just outside the metro Atlanta area wanted their 404 back. So the telephone company obliged. But this hastened the need for another area code. The 404 area had become, and to my knowledge still is, the world's largest toll-free calling zone. So back in August of 1995, new numbers in the surrounding cities of the Atlanta metro area were forced to use the 770 area code overlay. Then again in 1998, an overlay of both the 404 and the 770 went into effect. This was the area code 678. Three years later, 470 area code went over the top of them all: 404, 770, and 678.