"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 779 area code specific link here.
15 telephone companies servicing the 779 area code
List of telephone companies
TELEPORT COMMUNICATIONS GROUP - CHICAGO, BROADWING COMMUNICATIONS, LLC - IL, LEVEL 3 COMMUNICATIONS, LLC - IL, CELLCO PARTNERSHIP DBA VERIZON WIRELESS - IL, NEW CINGULAR WIRELESS PCS, LLC - IL, SPRINT SPECTRUM L.P., COMCAST PHONE OF ILLINOIS, LLC - IL, AT&T LOCAL, CHARTER FIBERLINK - ILLINOIS, LLC - IL, MTCO COMMUNICATIONS, INC., AMERITECH ILLINOIS, PAETEC COMMUNICATIONS, INC. - IL, T-MOBILE USA, INC., DIGITAL NETWORK ACCESS COMMUNICATIONS, INC. - IL, ESSEX TELCOM, INC.