Alarms can be setup as stand alone systems. To connect them into the telephone lines, it needs to be done correctly, or else someone could prevent the system from dialing out by simply taking a house phone off the hook. This trick is called line siezure, and is accomplished by circuitry built into nearly all modern alarm controls. The FCC and telecom providers require a means to disconnect alarm systems from the house wiring, for testing and repairs. This is accomplished using a RJ-31x jack and cord.
Source: http://forum.doityourself.com/electronic-alarms-home-security-devices/203784-alarm-forum-faq-please-read-before-posting.html