The KX-TGUA40B Additional Cordless Phone Handset is compatible with Panasonic KX-TGU4 series Cordless Phone Systems; DOES NOT WORK ON ITS OWN; MUST BE PAIRED WITH A COMPATIBLE PHONE SYSTEM. Add handsets to your phone (up to 6 total) to have landline-level call quality and advanced call blocking features at your fingertips anywhere in the home.