Siedle Compact audio set in 1+n technology with the functions of calling, speaking and door opening. Number of call buttons: 4 Each Compact set can be supplemented with an additional indoor station or a secondary signal device for each call button. The ZERT 811-. accessory is required for the storey call. Scope of delivery: . Siedle Compact audio door station (CA 812-4 E) . 3 standard in-house telephones (HTS 811-. W) . Transformer (TR 603-.)