RJ38X Pin Layout (Fire Alarm and Security Wiring)
The RJ38X jacks are almost identical to other USOC registered jack - RJ31X, also used for fire alarm and security equipment connections. The difference is in the short (strap) connection between pins 2 and 7 that lets the equipment, specifically designed to be plugged into a RJ38X jack, to probe if it has beet attached to the correct jack type.
The RJ38X jacks are designed to provide a fire or security alarm panel preference in dialing out in case of emergency. If any other terminal device (phone, fax, modem) is using the line when the alarm panel needs to call, the panel's internal circuitry breaks that connection and establishes its own. Do not plug any other device into the RJ38X jack: the jumpers between pins 1-4 and 5-8 are removed upon a 8P8C plug insertion, and the dial tone is not going to be delivered to house phones (trunk ports in case of aa KSU or PBX). The RJ38X jack has to be always located "upstream" from any other telephone equipment on the premise.
When a plug is removed, the house wiring gets connected to the phone company wiring through the 1-4, 5-8 jumpers, therefore it is not necessary to remove RJ38X jack if the fire alarm/security equipment has been uninstalled.