NXX

NXX is most commonly used to refer to three-digit code that forms the second part of a 10-digit North American phone number (digits 4-6). This is also known as the &quot;central office code&quot; or &quot;exchange&quot;. Traditionally, in the age of competing phone companies, each NXX is assigned to one company.

NENA 03-008, NENA Standard for E9-1-1 Default Assignment and Call Routing Functions