Busy Lamp Field (BLF) acting as a “digital presence” light:

BLF – the Busy Lamp Field acronym refers to a light on an IP Phone or a colored indicator next to a name in a softphone or web client. This indicator tells the operator the real-time status of other extensions or phone lines within the same system i.e. at a glance whether a coworker is available, on a call, or receiving a call.

Key Components of BLF:
The term “Lamp” (LED/Icon) refers to a light on an IP phone (or a color-coded icon on a softphone) that indicates the status of another user.
The term “The Field” refers to a designated, programmable button on a desk phone dedicated to monitoring a specific user.
The term “SIP Signaling” refers to the way that BLF works using SIP (Session Initiation Protocol) i.e. “subscribe” and “notify” messages. The phone asks the PBX for updates, and the PBX sends a notification whenever the status of a monitored extension change.

What BLF Status Indicators Mean
Color schemes can vary by phone manufacturer, but industry standard are:
Off/Green: The monitored extension is idle (available).
Solid Red: The monitored extension is busy (on a call).
Flashing Red: Extension is busy receiving incoming call (ringing).

Primary Uses of BLF
Call Transfer: Prevent call transfer to busy extension and going straight to voicemail.
Call Pickup: A ringing (flashing red) extension can be answered by pressing the BLF button.
Speed Dial: BLF button acts as a one-touch dial button to call an extension.
Presence/Monitoring: Quick visual of who is in a meeting, on the phone, or available.

Advantages
Improved Efficiency: Reduces the time spent transferring calls to busy lines.
Better Customer Service: Callers will not be put on hold as extension status are visible.
Workflow Coordination: Improves local/remote staff collaboration by knowing status.