said by IowaCowboy:
I can wire an RJ31X jack easily
The whole point of a RJ31X jack is to prevent the situation you describe. If the RJ31X jack is wired correctly it will be in between the NID and all
the phone outlets in the house. Therefore the alarm can seize the line regardless whether any phones are off-hook or not.
If the alarm itself has RJ-11 in and out connectors (instead of an RJ31X connector) you can buy an adapter or make one yourself:
RJ31X pins 4+5 go to RJ11 pins 3+4 for the alarm input/telco connector.
RJ31X pins 1+8 go to RJ11 pins 3+4 for the alarm output/home-phones connector.--
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