Door opening
From Zenitel Wiki
Door opening is a feature triggered by pressing digit 6 during conversation with the door station. When pressing the digit, a RCO is triggered for 3,5 seconds (programmable from AlphaPro, Exchange & System -> Systems -> Timers -> RCO Pulse (Door Open) Time). The RCO output is used to operate a relay on a MRBD board, to which the door magnet is connected.
