NEC DX4NA Door phone

The Door Box (P/N 922450 and BE106902) is a self-contained analog Intercom unit typically used to monitor an entrance door.

A visitor at the door can press the Door Box call button (like a door bell). The Door Box then sends chime tones or ringing to all extensions programmed to receive chimes.  To answer the chime or ringing, the called extension user just lifts the handset. This lets the extension user talk to the visitor at the Door Box. 

The Door Box is convenient to have at a delivery entrance, for example.  It is not necessary to have company personnel monitor the delivery entrance; they just answer the Door Box instead.