POTS Telephone System
An RJ-31X phone jack is based on the POTS telephone system. POTS stands for "Plain Old Telephone System". This is the two-wire telephone system that has been in use for nearly a century.
Normal-Through, Emergency Telephone Line Interrupter - Fire Alarm Telephone Capture
Fire alarm communication is supposed to take priority over all other communications. When there's a fire starting in a building and someone is gossiping over the phone line, or maybe when the fax machine is receiving a fax, calling for help is more important.
The automatic process with a fire alarm is to hang up on anyone else, then dial out and call for help.
With the POTS telephone system, picking up the telephone so it is "off-hook" is actually connecting the telephone to the phone system. Hanging up the telephone so it is "on-hook" is disconnecting the telephone from the phone system.
An RJ-31X Jack is a Normal-Through Jack
The RJ-31X jack normally passes-through the phone signals from the telephone company to the house phones. The jack can, if need be, interrupt the phone lines.
This is the common method of wiring the RJ-31X jack. The wires going to the telephone company (blue/white pair or red/green pair) are connected to the terminals 4 and 5 (the jack's red/green pair).
Inside the plug of the RJ-31X jack, there's a shorting bar. The wires going to the house phones (orange/white pair or black/yellow pair) are shorted to the terminals 1 and 8 (the jack's gray/brown pair).
Before connecting any wires or plugs to the jack, use your ohmmeter to confirm the two shorts caused by the shorting bar.
When the telephone wires are attached, but no dialing equipment is installed, this is the signal path.
Telephone Line Capture Relay
Inside the Fire Alarm Control Panel or Burglar Alarm Control Panel is a telephone line capture relay. The relay hangs up any house phone or fax machine, and then allows the control panel to dial out with the emergency message.
In normal times, the TIP and Ring terminal are connected to (or shorted to) the T and R terminals, allowing the house phones to be used normally.
When there's a cord plugged into the RJ-31X jack, the jack's shorting bar is pushed aside, allowing the signals to be routed through the cord going to other equipment, like the fire alarm control panel. The wires in the cord are usually the green/red pair.
The house phone is connected to pins 1 and 8 of the RJ-31X jack, go into the T and R pins on the Fire Alarm Control Panel, through the relay, out the TIP and Ring terminals of the control panel, and out pins 4 and 5 of the RJ-31X jack. The wires in the cord are usually the brown/gray pair.
When there's a fire alarm, supervisory, trouble, test, or other emergency, the control panel activates the relay, and then activates its internal dialer.
The relay, when activated, hangs up on the house phones by disconnecting them, and then passes through the emergency signals from the panel's internal dialer.