Note: Operator assistance or participation may be an optional function. In U.S. common carrier telephony service, a toll center designated "Class 4C" is an office where assistance in completing incoming calls is provided in addition to other traffic; a toll center designated "Class 4P" is an office where operators handle only outbound calls, or where switching is performed without operator assistance.
Source: from Federal Standard 1037C and from MIL-STD-188
wikipedia.org dumped 2003-03-17 with terodump