Line Seizure is something done with the wiring between the Alarm system, PBX, and the telephone line. In order to have line seizure, the phone line would have to go into the Alarm System, and then come out of the Alarm system into the PBX.
This is done with an RJ31x jack. Here is a guide to wiring a RJ31X jack:
http://www.hometech.com/learn/rj31x.html
One pair on the jack would be an input, and another pair an output. Connect the CO line to the input, and connect the PBX to the output.
Regards,
Adam Compton