Some IP phones have BLF function keys. The abbreviation BLF stands for Busy Lamp Field.
It is a flashlight on the IP phone, that shows whether another extension-
that is connected to the same PBX-busy or not. Depending on the type of the actual phone,
the light of the BLF remains green, that means the phone is not used at that moment.
If the BLF starts to flash in red, it means in normal case that someone is calling
that particular extension. If the BLF function key is red, it means that the
owner of the extension is in call.
BLF is also useful if the phone of another colleague is to be answered. For example,
if one of the colleagues is not at his/her desk and someone calls that extension,
another workmate can simply answer the call by clicking on the BLF. Furthermore,
before the colleagues want to call each other, they can check whether the other one
is in call or not.
Configuration of the BLF function keys
BLF function keys have to be configured manually. After the BLF function keys are configurated,
the IP phone subscribes for a source list that is available on the PBX.
This source list gives all the necessary information about the other extensions
on the PBX.
The BLF works via SIP protocol having used the Subscribe and Notify messages.
In normal case, the phone is the subscriber and the PBX is the notifier.
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