<P> As of 1995, with the introduction of interchangeable NPA codes, nearly all code combinations are useful as NPAs or as NXXes . </P> <P> Traditionally, identical central office codes in adjacent Numbering Plan Areas (NPAs) would be assigned as far apart from each other as possible, so that callers living near an NPA boundary would not confuse numbers in the adjacent NPAs . Central office code protection made it possible in some low density areas to use seven digits to reach areas in another area code . </P> <Ul> <Li> Washington, D.C. (area code 202) to adjacent areas of Maryland (area code 301) and Virginia (area code 703), discontinued on October 1, 1990 to allow assignment of a code in all three area codes . </Li> <Li> Hull from Ottawa before 2006, as every Ottawa - Hull local number originally was reserved in both area code 613 and 819). This was terminated on 21 October 2006 to allow assignment of a code in both 613 and 819, although five central office codes associated with the federal government were protected in 819 to allow dialing without potentially reaching non-government lines . </Li> </Ul> <Li> Washington, D.C. (area code 202) to adjacent areas of Maryland (area code 301) and Virginia (area code 703), discontinued on October 1, 1990 to allow assignment of a code in all three area codes . </Li>

When did phone numbers go from 6 to 7 digits