While 303 had been Colorado's sole area code for 40 years, the state's explosive growth in the second half of the 20th century made it apparent that the state would need another area code.
Colorado's reliance on 303 for its phone numbers, which had been in place for forty years, was no longer sufficient due to the state's rapid population growth during the latter half of the 20th century. This necessitated the introduction of a new area code.