It depends on what type of sensor and how you define "works".
If you arm your system to Away or Stay, the system will start to sound whenever a door or window contact sensor is opened. You should have an entry delay of 30-60 seconds to then enter your code to prevent an actual alarm being sent to our monitoring center. The same thing would happen if motion is detected on your motion sensor while the system is armed Away.
If the alarm code is not entered prior to the end of the entry delay, your siren(s) will be activated and our monitoring center will also be alerted. This could result in dispatch of your local authorities as well.
If your system is armed Instant, there is no delay. Your sirens will blare instantly and our monitoring center will be alerted if there is motion detected or if a window/door sensor is opened.
*I am an AT&T employee, and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.