Area Code 312-204 located in CHICAGO, COOK, IL, more detail is as below. Place NPA (Numbering Plan Area) : 312 NXX (Local Number Prefix) : 204 City : CHICAGO County : COOK State : IL (Illinois) NXX Use Type : Landline NXX Intro Version : 1997-10-11 FIPS (Federal Information Processing Standards) : 17031 LATA (Local Access Transport Area) : 358 Rate Center : CHICGOZN01 ZIP Code Count : 3 ZIP Code Frequency : 19 ZIP Code : 60614 Other Information NPANXX : 312204 County Population : 0 Switch CLLI (Common Language Location Identifier) : CHCGILSUDS1 MSA CBSA (Core Based Statistical Area) : Chicago-Naperville-Joliet, IL-IN-WI MSA CBSA Code (Core Based Statistical Area) : 16980 Time Zone : Central (GMT -06:00) Observes Daylight Saving Time : Yes Company : AMERITECH ILLINOIS OCN (Operating Company Number) : 9329 Coverage Area Name : CHICAGO Latitude : 41.8921 Longitude : -87.6311 ‹ previous : 312-204 next › : 312-204 Telephone Number List 312-204-0000 ~ 312-204-9999