Learned something new today:

* RJ31X: 8P8C (although usually only 4C are used), Often incorrectly stated as allowing alarm (fire and intrusion) equipment to seize a phone line, the jack is actually used to disconnect the equipment from the phone line while allowing the phone circuit to continue to the site phones.

* RJ38X: 8P8C, similar to RJ31X, with continuity circuit (this is the one that should be used for fire and security applications because of the continuity pair. If the plug is removed from the jack a short is created between pins 1 and 8 therefore if correctly wired will trip a panic or fire alarm.)

I use the 31X, with the 8 conductor cord.
4 conductors are the CO and the Premise line wiring. Then I place a resistor across 2 and 7 or 3 and 6, for 31X tampering(removal of cord), and adding a reed switch inside will also tell if someone removed the cover.
The remaining 2 conductors of the 8 can go out to D Mark, and beyond if possible to tell if phone line has been physically cut, and is an Instant loop. Most panels have telco monitoring and will sound a trouble if Line is lost, don't like the delay. Thanks

Nice idea. I like that.