Area code 312


Area code 312 is the telephone area code for downtown Chicago, which includes the Chicago Loop and its immediate environs. The plan area is completely surrounded by area code 773, which serves the rest of Chicago. Both area codes are overlaid with area code 872. 312 is an enclave area code, similar to area code 514 in Montreal and area code 210 in San Antonio. It is roughly bounded by North Avenue on the north, Western Avenue on the west, and 35th Street on the south, with Lake Michigan to the east. In terms of geographic area covered, 312 is one of the smallest area codes in the nation, encompassing only a few square miles.

History

Area code 312 was assigned to one of the original 86 numbering plan areas created in 1947 for the North American Numbering Plan. As a result of Illinois' dense population, particularly in the north, the state was split between four NPAs, given that each NPA could only accommodate approximately 500 central offices.
Area code 312 originally served almost all of the Chicago metropolitan area in Illinois, except for portions of Grundy, Kane, Kankakee, McHenry, and Will counties located in the numbering plan area of the rest of northern Illinois, 815. On November 11, 1989, the suburbs were assigned area code 708 and 312 was reduced to Chicago.
The 1989 split was intended as a long-term solution to relieve exchanges in one of the largest toll-free calling zones in the nation. However, within only five years, 312 was on the verge of exhaustion once again due to the proliferation of cell phones, pagers and fax machines in the 1990s. Additionally, the Chicago LATA extends into northwest Indiana, meaning several numbers in Indiana's 219 weren't available for use. This factor also forced 708 to be split in 1996, just seven years after it split from 312. 708 was retained for the southern suburbs, while the northern suburbs became 847 and the western suburbs became 630.
As a solution, all of the city outside of the downtown area was split off as area code 773 on October 11, 1996, making Chicago one of the few cities in the nation to be split between multiple area codes, along with New York City and Los Angeles. Since November 7, 2009, both 312 and 773 have been overlaid by 872. With the implementation of 872, even local calls in Chicago require 10-digit dialing, in addition to the prefix '1'.
The Illinois side of the Chicago area–312/773/872, 708, 847/224, 630/331 and portions of 815/779–is one of the largest local calling areas in the United States; with few exceptions, no long-distance charges are applied from one portion of the metro area to the other.
The first production #4ESS toll switch was installed in Chicago in January 1976.

In popular culture