Area code 646 is assigned to the borough of Manhattan in New York City. It was released in 1999 as an overlay to the 212 area code. An overlay occurs when a new area code is added to an area instead of splitting it into separate area codes.
AT&T assigned the 212 area code to the New York City in 1947. In 1984, Brooklyn, Queens and Staten Island were split off into the 718 area code, followed by the Bronx in 1992. Later that year, the entire city was overlaid with the 917 area code which, despite its intended use for mobile numbers, became necessary for residential use when 212 numbers became scarce. When the supply of 917 numbers ran out, the 212 area code was overlaid with the 646 code in 1999.