The home alarm system of ADT links to a monitoring center which notifies law enforcement of suspected intrusions or robberies. The monitoring centre then contacts you to let you know someone triggered the alarm. Thus, you do not need to call the police yourself. You should know that all the home alarms do not link to the monitoring center but ADT is linked to the monitoring system. There are some monitoring systems that cannot have the link system and these are only noise makers. Loud, noisy alarms are advantageous because they give you an expedient heads up, possibly providing valuable time for you to exit your home and get to a safe location to call for help.
In modern days you can also install modern wireless alarm system. Both of these systems have positive and negative sides and you know that the disadvantage of wireless system is that it requires batteries and won't work once the batteries die. It will not work if you do not check the batteries regularly. The New York ADT has also wireless system and the alarm must be installed close to a central control unit, so there are limits as to where it effectively can be located at anytime at your convenience.