Direct-Call Audio Door Phone

1) 30s sixteen-chord melody ring, 15s monitor if needed, 120s confidential conversation 2) Three kinds of ring tone available to easily identify different door station 3) Three-level volume adjustable 4) Broadcast calling and intercom function 5) Flush-mount for door station 6) 4 wires polar connection among door station, main monitor and monitor 7) Trigger signal to unlock (workable with various locks)