Area codes 301, 240, and 227

This includes the communities of Cumberland, Frederick, Hagerstown, Gaithersburg, Potomac, Germantown, Bethesda, Rockville, Landover, Silver Spring, and Waldorf.

By the end of the 1980s, the Washington metropolitan area was running out of assignable prefixes for new central offices on both sides of the Potomac River.

The only unassigned prefixes were unavailable due to the central office code protection that maintained seven-digit dialing in the metro-area.

While Maryland would have likely needed another area code due to the growth of the Baltimore-Washington corridor, it is likely that a split would have been delayed had more 301 numbers been available for use in Baltimore.

Although the area code split was intended as a long-term solution, within four years 301 was close to exhaustion due to the proliferation of cell phones and pagers, especially in the Washington suburbs.

[11] A September 2022 study projected that the 301/240 numbering plan area (NPA) would suffer central office code exhausted between April and June 2023.

Some areas, such as Rockville,[16] Gaithersburg,[17] Upper Marlboro,[18] Bethesda[19] and Landover in Maryland[20] remain a local call to the District and Northern Virginia.