BLF means Busy Lamp Field. Generally, this is an LED that shows the status of an extension or line, an application, or even the presence of a user.
Sometimes, this LED is physically embedded into a key that when pressed, can enable or disable an application, and depending on these states, the LED can be turned off or on.
In most of the cases, the BLF points to an extension. For example, an assistant can notice if the boss is busy (LED on) or free to be called (LED off). All IP phones support BLF in most cases.
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