"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 939 area code specific link here.
CENTENNIAL PUERTO RICO OPERATIONS CORP. - PR, TELEFONICA LARGA DISTANCIA DE PUERTO RICO - PR, PR WIRELESS, INC. DBA OPEN MOBILE, NEW CINGULAR WIRELESS PCS, LLC - GA, CENTENNIAL PUERTO RICO OPERATIONS CORP., PUERTO RICO TELCO DBA VERIZON WIRELESS PUERTO RICO, SPRINT SPECTRUM L.P., PUERTO RICO TELEPHONE CO.