Simply put, line seizure ensures the fact that when the Security System needs to communicate with the Central Monitoring Facility, all other phone line activity will be temporarily interrupted to allow the Security System to report the current alarm or maintenance condition that exists. 

This means that the path of the dial tone flows from the interface into the alarm panel, then back to the phone wires inside of the interface that feed the household phones. Only by placing the alarm panel in series with the phone line can you obtain proper line seizure.

What can happen if the homeowner or a phone company technician changes the connection that the alarm company makes when the alarm system is first installed?

There are several things that can occur when the homeowner or the phone company does any new work inside of the telephone network interface. Many times, the telephone company technician may not be familiar with the type of connection that has been made to connect the alarm system to the telephone line. Upon troubleshooting a problem on the line, or adding a new extension, it is quite common for the wires feeding dial tone to the alarm system to become disconnected.

It is also quite common to see additional extensions added by bringing new wires to the interface, and connecting the new service onto the screw terminals, in parallel to your alarm panel, eliminating line seizure as you originally installed it.

Most newer alarm panels will have some sort of trouble indicator due to this loss of dial tone, but usually the homeowner doesn't notice it until the next time that they arm the system, usually after the phone company has left the premises.

Another situation could occur if the red and green "feed" wires have been reconnected to the alarm system, in parallel with all of the other telephone service. On most panels, this will defeat the "trouble" circuitry and will still communicate if an alarm occurs, as long as no other phones are in use. When the alarm is simply connected in parallel with the rest of the lines, the alarm system is no longer capable of "seizing the line" during an alarm condition. Eventually, the customer will contact the Alarm Company due to the lack of response from the central station, but of course, if there had been an actual break-in, or fire, the damage has already been done.

As alarm company owners, installers, and technicians, over the past 12 years, we have gone out on many service calls due to problems with the telephone line connection to the alarm panel. We have always utilized high standards for all of our installations, making our phone line connection in the network interface, and not merely cutting into a telephone wire running through the attic or basement.

We always explain to the customer that the alarm system's ability to transmit signals to the Central Station system depends on the proper connection to the telephone line. Even though we always tell our customers to check their keypad after any work has been done in their home, many problems have occurred due to other workers or repairmen working in their home or business. Things like disconnecting the AC transformer to plug in a drop light to do work in the basement, or door and window installers simply cutting wires leading to door and window contacts are somewhat common, but by far, we have experienced the most problems stemming from the routine disconnection of the alarm/phone line connection many times when the customer actually had a problem that was completely telephone related, and had nothing to do with the alarm.

The fact that most newer security systems have a trouble indicator on the keypad only seems to make the customer more likely to think that their problem with the phone line has something to do with their security system, when if fact, it is the opposite, and the alarm system is simply alerting them to the fact that their phone line is not in service for some reason.