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Area codes 303 and 720 are telephone area codes serving the north-central portion of the state of Colorado. They cover Denver, Lakewood, Littleton, Boulder, Longmont, Aurora, Castle Rock and most of the Front Range. Area code 303 is the main 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.