In the traditional public switched telephone network (PSTN) hierarchy, a primary toll center. Class 3 offices served to connect Class 4 offices for intrastate toll calling, and to interconnect independent telcos and the Bell operating companies (BOCs). Approximately 200 Class 3 offices existed prior to the breakup of the Bell System. It is doubtful that any remain. See also Bell System, BOC, Class 4, independent telephone company, and PSTN.