Compact internal audio unit for surface mounting

This compact internal audio unit is delivered standard equipped to perform the following functions: allow a conversation with a visitor, produce a different tone for a call at an apartment door, main door or an internal call, switch lights on and off, open doors, adjust the volume, switch off the ring tone, indicate a busy signal and call signal with LEDs. The controls consist of 5 function keys and a volume adjustment. The functionality of one of the keys can be freely assigned. This programmable key can be set up to either transmit a control function for relay 10-831, allow a parallel call, make an internal call, set up a ring-and-enter function, or activate a call redirection command. The communication with the visitor is hands-free.