In telephony, an ERC is an easily-recognizable code for the North American Numbering Plan. This applies to all area codes that end in double digits, such as 800, 877, 900, 700, and others not yet assigned. The state of Nevada requested that its new area code be "lucky" 777, but this was denied by NANPA because it is an ERC, to be used only for special purposes like inward WATS.
