The V-2900 Door Answering Unit provides voice contact from a designated telephone station to a door. When used in conjunction with a door plate speaker, pressing the pushbutton on the door plate speaker will cause the phone to ring (1 second on, 4 seconds off). By answering the call, the system user can converse with the person at the door. (The telephone may also call the door speaker). The V-2900 can be used with a dedicated single line telephone set, a loop start trunk port of a PABX or C. O. Line Position of an E-Key System.