 After World War II, the newly conceived North American Numbering Plan of 1947 sought to unify all local numbering plans by using a system of two central office letters and one digit to complete the office prefix, and four digits for line number.This system was referred to as "2L-5N", or simply "2-5".With increasing demand for telephone service in the 1950s and 1960s, resources were strained not only in employing telephone operators, and for the network system of central offices, but also for the traditional numbering plan using telephone exchange names.