What the alarm panels monitor is Voltage AND Current.
Since an RJ-31 jack routes the phone line THROUGH the alarm circuity, it is a both/and situation.
If you go off hook AHEAD of the RJ-31 jack, there will be a voltage drop, but NO current flow THROUGH the alarm circuitry, thus it will trip.
If the phone line is cut, again there will be a voltage drop (zero volts), and NO current flowing through the alarm circuitry, the alarm will trip. If it is a Fire Alarm, there will be a second line that it will use to report the trouble condition.
If you go off hook AFTER the RJ-31 jack, there will be a voltage drop, AND there will be current flowing through the alarm circuitry, and it will be happy with that.
So the alarm just needs to be set for the typical ON-HOOK voltage, usually -48VDC, but could be -24VDC.
