COMMAND COMMUNICATIONS FONE SITTER, OFF THE PHONE PHONE ALERT, SAFETY ALERT ONLY WORKS FOR LANDLINES. DOES NOT WORK WITH VOIP OR CORDLESS PHONES. Automatically alerts you to any extension phone left off-hook throughout your home and in addition alerts you if your phone service has been interrupted Just one unit will monitor one phone line for your entire house Requires 2 AAA batteries, not included 2 Different audible alarm tones (one for off-hook and the other tone for service interrupted)