 Area code 303 is the original area code, while area code 720 is an overlay, meaning that the same physical service area is served by both area codes (303 and 720) and that ten-digit dialing is required for all calls made within the service area.Area code 303 was one of the original area codes put into service in 1947, and originally covered the entire state of Colorado.Despite Colorado's dramatic growth during the second half of the 20th century (especially in the Denver area), it remained the sole area code in Colorado for over 40 years.