Siedle Basic audio set in bus technology with the functions of calling, speaking and door opening. Number of call buttons: 4 Each basic set can be expanded to include selected door and indoor stations, as well as switching functions and a telephone connection. Scope of delivery: . Siedle Basic audio door station (CA 850-4 E) . 4 Siedle Basic audio indoor stations (AIB 150-.) . Set power supply unit (SNG 850-.)