http://en.wikipedia.org/wiki/Area_codes_212_and_646

sidenote: in reading that entry
found out about Marble Hill which is crazy:

"Marble Hill
One Manhattan neighborhood, Marble Hill, is not in the 212/646 area code but the 718/347/929 codes. Marble Hill, although legally a part of Manhattan to this day, was geographically severed from Manhattan by the construction of the Harlem River Ship Canal in 1895. It was physically connected to the Bronx in 1914 when the by-passed segment of the Harlem River was filled in. When the Bronx shifted to 718 in 1992, Marble Hill residents fought to stay in 212, but lost. Marble Hill's trunk is wired into the Bronx line, and it would have been too expensive for New York Telephone to rewire it."