This device can be added to an internal unit as an extra bell so that a call at the door never goes unheard. There is a different tone for a call at an apartment door, the main door or an internal call. The user can easily switch off the extra bell. The ring tone volume can be adjusted at the time of installation. The device is connected to the bus system and can be programmed to work with a bell push button or in parallel with 2 internal units. It is not necessary to add an extra contact or bell transformer.