Telephone switching system incorporating selectively controlled ringback

Claim 1

A TELEPHONE SWITCHING SYSTEM C OMPRISING A TELEPHONE CENTRAL OFFICE, A PLURALITY OF REMOTE SWITCHING UNITS, A CONTROL UNIT COMMON TO SAID SWITCHING UNITS, A DATA TRUNKS FOR CONNECTING SAID CONTROL UNITS TO EACH OF SAID SWITCHING UNITS, A PLURALITY OF EXTENSIONS TERMINATING AT EACH OF SAID SWITCHING UNITS, A PLURALITY OF CENTRAL OFFICE TRUNKS TERMINATING AT EACH OF SAID SWITCHING UNITS AND SELECTIVELY CONNECTABLE THROUGH SAID SWITCHING UNITS TO SAID EXTENSIONS, AND SIGNALING TRUNKS INTERCONNECTING EACH OF SAID SWITCHING UNITS AND SAID CONTROL UNIT FOR TRANSMITTING SIGNALING INFORMATION FROM SAID SWITCHING UNITS TO SAID CONTROL UNIT, MEANS FOR DETECTING A PREDETERMINED SUPERVISORY CONDITION EXHIBITED BY CALLS EXTENDED BETWEEN SAID EXTENSIONS AND SAID CENTRAL OFFICE, MEANS IN SAID CENTRAL OFFICE TRUNKS FOR CONVERTING RINGING SIGNALS FROM SAID CENTRAL OFFICE TO SIGNALING INFORMATION RECOGNIZABLE BY SAID CONTROL UNIT, MEANS IN SAID CONTROL UNIT FOR GENERATING AND TRANSMITTING CONTROL DATA OVER SAID DATA TRUNKS TO CONTROL SAID SWITCHING UNITS TO CONNECT THE CENTRAL OFFICE TRUNKS SERVING CALLS EXHIBITING SAID PREDETERMINED SUPERVISORY CONDITION TO SAID SIGNALING TRUNKS, AND MEANS IN SAID CONTROL UNIT FOR DETECTING THE APPERANCE OF SAID CONVERTED RECOGNIZABLE SIGNALING INFORMATION ON SAID SIGNALING TRUNKS.