What is a BLF? Busy Lamp Field, this is a light on the IP Phone that tells you whether another extension connected to the same PBX is busy or not.
If you have a Yealink IP Phone, these are what the light colors would indicate:
Green light - Available
Fast blinking Red light - Extensions is ringing
Red Light - Busy
Slow blinking Red Light - Other party is On HOLD
To add an extension for BLF, you would need to log in to the Web Settings for the specific phone. You would need to get the IP address of the specific unit - Please see https://help.gridare.com/index.php?/Default/Knowledgebase/Article/View/141/0/how-to-check-ip-address-on-a-yealink-phone to get the IP address.
Type the IP address on your browser.
UN/ PW would be admin/ admin.
Once you're logged in, go to DSS Key tab:
You would be able to decide which Line key you would want to place the extension on depending on which one is available on your screen:
Once you've decided on it, You would need to choose the Line Key tab, found on the left side of your screen.
On the photo below - it is on Line Key 19-27 and the user has chosen Line Key 19 to place the extension on. 
You can also choose through the drop down key if you would want to use BLF, Speed Dial, etc.
When you are done. Press Confirm/ Save
