Size: 21 keys, 122mm long x 77mm wide x 48mm thick Source: 12V DC Part address: 1 set of 8-bit DIP switches, address value 1-255; double call button Screen: 6 digits LCD display, 2 digits showing the number of people waiting, 4 digits showing the current call number Signal transmission: wireless/wired transmission, wireless built-in 433MHz wireless module