One page per area code: where it is, how its numbers are written and validated, and how its exchanges (the three digits after the area code) were allocated, by carrier and by line type. 408 codes and 232,006exchanges on record; a code gets a page once at least 25 of its exchanges have a carrier and a line type on record. The +1 territories with their own numbering plan page — Puerto Rico, Guam and the others — are under countries.