Apparatus for combining a central telephone trunk line with a customer telephone trunk line

In a system for combining a central telephone trunk line of a central telephone exchange with the trunk line of a customer exchange having power feed, ring trip and dial pulse functions, wherein said central telephone exchange is provided with means for separating the dial, talk, and ring functions into a first partial circuit for providing a service signal to and from said customer trunk line, a second partial circuit for providing a ring signal to said customer trunk line, and a third partial circuit for voice transmission operable by completion of said first partial circuit, an interface system comprising a first circuit connected to said first partial circuit for controlling the operation of each of the power feed, ring trip and dial pulse of said customer exchange, a second circuit connected to said second partial circuit for controlling the ring signal from said telephone exchange, and a third circuit connected to said third partial circuit, a first relay located in a said second circuit having a winding connected between a DC source of current and ground, said first relay having a normally closed contact located in said third circuit operable on activation of said first relay to connect a lead of said customer trunk line to a source of AC ring current, a second relay located in said third circuit having at least two windings and a single core, said relay being operable by a current through either of said windings, one of said windings being connected between a DC source of current and one lead of said customer trunk line, the other of said windings being connected between ground and the other lead of said customer trunk line, said second relay having a normally open first contact in said first circuit operable on looping of said customer trunk line to complete said first partial circuit and establish service with said telephone exchange, and a normally closed second contact in said second circuit operable on looping of said customer trunk line to disconnect said first relay from said DC source of current and discontinue the AC ring current.