In addition to 303 and 720, the Denver metro will soon be getting a new area code: 983. Colorado's population is growing and we're running out of phone numbers! By the end of next year they expect that the remaining 303 and 720 area codes will be gone, so a third will be added in April of 2022. This new area code will serve central Colorado including Denver, Arapahoe, Jefferson, Adams, Boulder and Douglas counties.
Of course 303 was Colorado's original area code established in 1947 but has been split into different area codes several times since then. Read more here.