A passive infrared sensor (PIR) is waiting for a person to enter a room. If some movement is detected, it reports to a pic microcontroller. Finally the pic dials a phone number through the modem to alert the user.
What you can see in the video is: The modem is turned on and the pic configures it to disable command echo, change to return numeric responses and turn off the volume of the modem speaker. The user has 15 seconds to leave the room, then the pic waits a motion detection. When it is detected the pic dials a fixed phone number and the user will receive the alert.
[Source] Hack-A-Day via O.R.P SYSTEMS