Avaya definity Operation Manual page 29

Selector console
You can determine the status of an extension by pressing the HGS button and
looking at the lamp to the left of the appropriate DXS button.
If the lamp is dark, the extension is idle.
If the lamp is lighted, the extension is in use. You can still transfer a call if
system users have more than one line available. Ask your system manager
about the phones in your company. If the extension is busy, you hear a busy
tone.
If a station has the SAC feature active and is busy, the DXS/BLF lamps
remain steady until the station is no longer busy. At that time, the lamp
goes to a flashing mode.
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