Electronic private branch exchange

Claim 1

AN ELECTRONIC SWITCHING TELEPHONE SYSTEM COMPRISING A CURRENT CONTROLLED, SELF-SEEKING SWITCHING NETWORK, A PLURALITY OF TELEPHONE SUBSCRIBE LINE CIRCUITS CONNECTED TO CERTAIN ACCESS POINTS IN THE NETWORK AND A PLURALITY OF LINK CIRCUITS AND TRUNK CIRCUITS CONNECTED TO OTHER ACCESS POINTS IN THE NETWORK, MEANS COMPRISING SAID TRUNK CIRCUITS FOR GIVING ACCESS TO AND EXTENDING TRUNK CALLS THROUGH SAID NETWORK, MEANS OPERATIVE ON A TIME SHAFTING BASIS FOR ASSIGNING SAID LINK CIRCUITS AND SAID TRUNK CALL ACCESS MEANS TO CONTROL THE EXTENSION OF SWITCH PATHS THROUGH SAID NETWORK, MEANS RESPONSIVE TO A TRUNK CALL COMPLETION CONDITION FOR CAUSING SAID TRUNK CALL ACCESS MEANS TO EXTEND A TRUNK CALL THROUGH SAID NETWORK DURING THE PERIOD OF SAID TIME SHARING WHEN SAID TRUNL CALL ACCESS MEANS IS ASSIGNED BY SAID ASSINGING MEANS, EACH OF SAID TRUNK CIRCUITS COMPRISES MEANS FOR INTERCONNECTING SAID ELECTONIC SYSTEM AND AN ELECTRO-MECHANICAL SYSTEM INCLUDING MEANS FOR SELECTIVELY SENDING OPEN LOOP OF CLOSED LOOP SIGNALS FROM SAID TRUNK CIRCUIT OVER A TRUNK LINE TO SAID ELECTROMECHANICAL SYSTEM, MEANS FOR COMPLETELY ISOLATING SAID LOOP SIGNALLING MEANS FROM SAID TRUNK LINE WITH RESPECT TO D.C. POTENTIALS IN SAID ELECTRONIC SYSTEM, AND MEANS FOR TRANSMITTING AT LEAST VOICE FREQUENCY A.C. SIGNALS BETWEEN SAID TWO SYSTEMS VIA SAID LOOP SIGNALLING MEANS.