Single indoor station for expanding the Vistus door intercom system, item numbers 515975, 515977, and 515978. Visual doorbell indicator: blue light frame around the talk and door opener button. The doorbell volume is infinitely adjustable. You can mute the doorbell sound using the silence button (e.g., during nighttime) - a red light frame blinks as a reminder. With intercom function: communication from indoor station to indoor station within a residential unit. Only 22 mm flat surface-mounted installation. Connection for floor doorbell button with its own doorbell sound to distinguish between house and apartment doors.