The TDU-4000 provides protection for phones from being used as clandestine listening devices. The unit electrically disconnects the telephone line from the phone when the phone is in the “on hook” position. During disconnect the telephone line is left open, while the voice path on the phone is grounded. This disables room audio from being transmitted onto the telephone line to prevent any eavesdropping. The phone will only connect to the telephone line when it is in the “off hook” position. The “Off Hook” detect and “Ring” detect signals are also opto-isolated from the phone.

A typical link consists of one TDU-4000 with connections to the POTS line and to the subscriber’s phone. FiberPlex recommends “R” units for high security applications because the units have added shielding and filtering for RFI suppression.