What is BLF?

BLF is an acronym for Busy Lamp Field, which is a light on an IP Phone or a colored indicator next to a name in a softphone or web client, that tells you whether another extension connected to the same PBX is busy or not.

Many popular IP phone manufacturers, such as Yealink, Fanvil and Snom, sell a variety of phones that have BLF function keys. Depending on the type of phone you have, BLF’s will either been green, flashing red, or red. If a BLF key is green, that means the extension number is free to talk. If the BLF starts to flash red, it usually means someone is calling that extension. If the BLF function key is red, it means the owner of that extension is on a call or unavailable.

BLF’s are also helpful when answering another colleagues phone. For instance, if a colleague isn’t at their desk and they receive an incoming call, someone else can pick up the call simply by clicking the flashing BLF. Additionally, you can configure BLF keys so that they are associated with the extensions of your choice. This works well for the colleagues that you contact the most. Not only can you monitor who is available, but you can also use the keys as a speed dial function for your most commonly dialed extensions.

BLF function keys can be configured really easily using the 3CX Management Console. Once the BLF function keys have been configured, the IP Phone subscribes to a resource list that’s available on the IP PBX. This resource list gives the phone all the necessary information about all the other extensions on the phone system.

It works through the SIP protocol by making use of the subscribe and notify messages. In a normal scenario, the phone is the subscriber and the IP PBX is the notifier. If you’d like to take an in-depth look at BLF function keys and the subscribe process, check out the Busy Lamp Field (BLF) blog post.

For step-by-step instructions on how to setup BLF keys on phones supported by the 3CX VoIP solution, consult our detailed configuration guides.

It’s not only IP phones that offer BLF functionality. Some modern softphones also feature BLFs, like that of the 3CX app for Windows. These digital BLF keys offer the same functionality as IP phone BLFs, however, a user can usually assign more BLF keys to the desired extensions – with 3CX they are unlimited.

BLF keys also allow employees to quickly and efficiently perform other tasks such as call transfers. Watch our video on how to perform an attended transfer using BLF module keys.