"E" is Eastern Time Zone, "C" is Central Time Zone, "M" is Mountain Time Zone, and "P" is Pacific Time Zone. Occasionally, an area code will fall in more than one time zone. In this case, the abbreviation is a combination of the previous abbreviations. Some area codes are shown with time zones referencing Universal Coordinated Time (UTC).
Additional technical status can be retrived from the North American Numbering Plan Administration on the 312 area code specific link here.
Area code 312 went live into service on January 01, 1947 (The paperwork process started on January 01, 1947).  It has had the 708, 773, 872 area code(s) overlaid on it.
This original "parent" area code went live on January 01, 1947 and due to growth it has been "overlaid" by the 872 area code to provide more telephone numbers in the area. This means this original "parent" 312 area code has seen quite a bit of growth and as a result has been subdivided into 2 area codes over the years. Planning is underway and notices have been distributed regarding the area code subdivision.
CHICGOZN01, NORTHBROOK, WHEELING, ROSELLE, CHICGOZN07, LA GRANGE, HINSDALE, CHICGOZN04
Number of major cities served
Names of major cities served (ZIP code)
(00000), Chicago (60606), Chicago (60610), Chestnut Street (60610), Fort Dearborn (60610), Chicago (60616), Twenty Second Street (60616), Northbrook (60062), Chicago (60605), Wheeling (60090), Chicago (60607), Chicago (60601), Roselle (60172), Keeneyville (60172), Chicago (60637), Jackson Park (60637), La Grange (60525), Countryside (60525), Hodgkins (60525), Ind Head Park (60525), Ind Head Pk (60525), Indian Head Park (60525), Indian Head Pk (60525), La Grange Highlands (60525), Lagrange Hlds (60525), Mc Cook (60525), Indianhead Park (60525), Indianhead Pk (60525), La Gran Hghls (60525), Mccook (60525), Hinsdale (60521), Oak Brook (60521), Oak Brk Mall (60521), Oak Brook Mall (60521), Chicago (60604), Arlington Heights (60004), Arlington Hts (60004), Chicago (60647), Roberto Clemente (60647), Chicago (60602)
35 telephone companies servicing the 312 area code
List of telephone companies
AMERITECH ILLINOIS, NEW CINGULAR WIRELESS PCS, LLC - IL, ALLEGIANCE TELECOM, INC. - IL, CELLCO PARTNERSHIP DBA VERIZON WIRELESS - IL, LEVEL 3 COMMUNICATIONS, LLC - IL, XO ILLINOIS, INC., UNITED STATES CELLULAR CORP. - ILLINOIS, BROADWING COMMUNICATIONS, LLC - IL, AMERICAN MESSAGING (AM), INC., PAETEC COMMUNICATIONS, INC. - IL, AT&T LOCAL, TELEPORT COMMUNICATIONS GROUP - CHICAGO, GLOBALCOM, INC.-IL, USA MOBILITY WIRELESS, INC., MCIMETRO ACCESS TRANSMISSION SERVICES LLC, NEXTEL COMMUNICATIONS, INC., ACCESS ONE, INC. - IL, GLOBAL CROSSING LOCAL SERVICES, INC.-IL, SPRINT SPECTRUM L.P., SIGECOM, LLC - IL, NEUTRAL TANDEM-ILLINOIS, LLC - IL, MCI WORLDCOM COMMUNICATIONS, INC. - IL, RCN TELECOM SERVICES OF ILLINOIS, INC. - IL, T-MOBILE USA, INC., CBEYOND COMMUNICATIONS, LLC-IL, OVATION COMMUNICATIONS OF ILLINOIS, INC., DENALI SPECTRUM LICENSE, LLC, AT&T - LOCAL - IL, YMAX COMMUNICATIONS CORP. - IL, PG TELECOM OF ILLINOIS INC. - IL, CIMCO COMMUNICATIONS, INC. - IL, CELLULAR PAGING USA, INC., GLOBAL NAPS, INC. - IL, INDIANA PAGING NTWK, MULTI COMPUTER SYSTEMS, INC.