When TeleData Guard is installed on all your extensions, the extension you pick up automaitcally disconnects the others! No more running to turn off the answering machine. LED also tells you - before you pick up - if the line is in use. Like a mini-phone system, it protectects your fax or modem from interruption, saving you steps, and insuring your privacy. If during a phone conversation the green light goes out, someone has activated a tap or picked up an extension. Press the red button to resume talking, now with caution. Measures just 3-1/2"X2"X1" and requires no batteries.