Area codes 410, 443, and 667 are telephone area codes serving the eastern half of the U.S. state of Maryland, including the Baltimore metropolitan area and the Eastern Shore. The 410 area code is the original area code, while the 443 and 667 codes are overlay codes. 443 and 667 were primarily used with cellular phones and competitive local exchange carriers such as Comcast or Cavalier Telephone when introduced but have since become universal in their carrier availability. History Before these area codes were created, all of Maryland had been served by 301 since the institution of area codes in 1947, even though the state is home to two very large metropolitan areas—Baltimore and the Maryland suburbs of Washington, D.C. (area code 202). This made Maryland one of the most-populous states to be served by a single area code. However, by the late 1980s, 301 was on the verge of exhaustion due to the rapid growth of the Baltimore and Washington suburbs, as well as the proliferation of fax machines and pagers. The number shortage problem was exacerbated by the use of 202 as a de facto overlay for the inner ring of the Washington metro area, even though it was split between three area codes–301, 202, and Northern Virginia's 703. This was accomplished via a system of central office code protection in which no central office code was duplicated in the region. Each existing central office code was properly routed with each area code in the region so that each telephone number in the region could be dialed with any of the regional area codes, giving the appearance of an overlay plan. One consequence of this was that a central office prefix could not be duplicated anywhere in the Washington area. Thus, if a 202-574 number was in use in the District or a 703-574 number was in use in northern Virginia, the corresponding 301-574 number could only be assigned to an area considered a safe distance from Washington, such as but not limited to in the Eastern Shore. The partial overlay ended in 1990, but it soon became apparent that this would not free up enough numbers to meet demand. By the fall of 1990, it was apparent that Maryland needed another area code. In November 1990, a plan to add a second area code to the state was announced. It was decided that the Baltimore metropolitan area