Telephone Wiring Modes
Telephone wiring modes refer to the wiring configurations used to connect auxiliary apparatus such as alarm panels, fax machines, data equipment, auto dialers, etc to an existing telephone service. Mode 3 is by far the most commonly used configuration with 611 sockets commonly (and erroneously) often referred to as a Mode 3 socket.
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MODE 3

Note 1:
If an earth is required for auxiliary equipment it is connected as shown thus --x-x-x-x-
Note 2.
Connection to these terminals is required only where the auxiliary
equipment incorporates facilities for switching the line to the
telephone.
Note 3.
Where the telephone is connected to a 4 wire extension from a PABX,
terminate the fourth wire on terminal 4 of the 611 socket.