"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 331 area code specific link here.
14 telephone companies servicing the 331 area code
List of telephone companies
AMERITECH ILLINOIS, COMCAST PHONE OF ILLINOIS, LLC - IL, SPRINT SPECTRUM L.P., CELLCO PARTNERSHIP DBA VERIZON WIRELESS - IL, DENALI SPECTRUM LICENSE, LLC, T-MOBILE USA, INC., BROADWING COMMUNICATIONS, LLC - IL, NEW CINGULAR WIRELESS PCS, LLC - IL, PAETEC COMMUNICATIONS, INC. - IL, US XCHANGE OF ILLINOIS LLC DBA CHOICE ONE COMM IL, GLOBAL CROSSING LOCAL SERVICES, INC.-IL, BANDWIDTH.COM CLEC, LLC - IL, MCI WORLDCOM COMMUNICATIONS, INC. - IL, SIGECOM, LLC - IL