720 Area Code – Denver, Colorado: Location, Time Zone & History

Denver and Boulder had already watched their original statewide code, 303, get carved up twice -- losing the southeast to 719 in 1988 and the north and west to 970 in 1995 -- before area code 720 arrived on September 1, 1998. Rather than another geographic split, 720 was layered directly on top of the shrunken 303 territory as an all-services overlay, the first Colorado had ever used, and it brought mandatory ten-digit dialing to the metro area the same day.

The overlay approach meant 303 and 720 numbers would forever mix within the same Denver-Boulder footprint, and callers have used them interchangeably ever since. A second overlay, 983, followed on June 17, 2022, after approval by the Colorado Public Utilities Commission the year before, stacking three codes onto one region now estimated at around 3.5 million people across Denver, Boulder, Aurora, Lakewood, Littleton, Longmont, Broomfield, and Castle Rock.

The 303/720/983 complex sits at the center of Colorado's technology, aerospace, telecommunications, and finance sectors, alongside a steady tourism trade drawing on the Front Range. Its rapid growth in landlines, cell phones, fax machines, and pagers through the 1990s is precisely what made an overlay necessary in the first place.