What is BLF?
When you have a customer on the phone and you want to transfer him/her to a colleague, it is nice to be able to see whether that colleague is available at that moment. That saves time and frustration! With the Busy Lamp Field (BLF) you can see exactly who is available, who does not want to be disturbed and who is busy. This saves you and your customer a lot of time, because you can inform your customer right away about the availability. 
With BLF on your VoIP phone, you can see if a user or colleague is busy or available: a light will blink green, orange or red. Especially convenient for companies with several offices or locations.
On Yealink VoIP phones the BLF buttons even have two functions. You cannot only see if a phone is available, you can also use the button as quick dial to that phone. If a call comes in for your colleague, you can also pick up his phone for him. This is called Call pick-up. 
For some phones it is possible to extend the nummber of buttons. Sets for Receptionist phones are also available and you can even extend to 240 BLF-buttons.
Set up BLF
On the phone
You can set up BLF onthe phone. You can read how this works for Yealink, Snom and Cisco-phones in this knowledgebase-article.
Via Auto provisioning
It is convenient to set up BLF via auto provisioning. Read about how provisioning works or where you can add the text to the custom config-files.
Snom: Add the following text to your custom config-files.
**#AsteriskDisabledsharedYOURSHORTNAME<Extended_Function_4_group=”Phone/Line_Key_4″>fnc=blf+sd+cp;sub=EXT#@PROXY;
- Change YOURSHORTNAME to your own short name, This will be the name shown next to the light.
- Change EXT# in the extension you want to be able to see.
- Change PROXY into the IP-address of your PBX.