How to Use BLF with S-Series VoIP PBX

What is BLF?

Busy light field (BLF) is typically a collection of lights or indicators on a phone that indicate the status of the lines or other phones connected to the same Yeastar S-Series VoIP PBX. IP phone can subscribe the status of other extensions or trunks via BLF. With this feature those real-time status can be presented at DSS keys in IP phone (In this guide, we mainly take Yealink IP phones as example).

It is important to note that this feature requires the support of both IP phones and IPPBX.
IP phone which is interested in "PSTN lines" or "other extensions" status sends a SUBSCRIBE message to the Yeastar S-Series VoIP PBX for subscription authorization. Once subscription authorized Yeastar S-Series VoIP PBX will send NOTIFY message to the IP phone every time the subscribed target status has been changed.

Light of the IP phone’s DSS key Status of the lines or extensions subscribed
Green IDLE
RED In use
Red flashing Extension is Ringing

Note: This guide applies to S-Series (Firmware version or later).


How to use BLF?

Firmware Version:

  • Extension Status Monioring

In the following example, we all take Yealink T26 (version as example.

Path: Login phone web GUI> DSS key


Type: Choose BLF mode

Value: Input the extension number or trunk name of Yeastar S-Series VoIP PBX you want to monitor.

Line: Choose the account number, which is registered to Yeastar S-Series VoIP PBX.

Extension: Input the feature code so that when this extension is ringing, you can press this key to execute an opeartion. In this example *04 means picking up the extension filled in the Value field when it is ringing.


  • Pick up Calls by BLF

We could subscribe the status of Yeastar S-Series VoIP PBX extensions. For example, when extension 705 is ringing, we can press the DSS key 1 to pick up the incoming call. The pre-configured *04 will be sent to to do the call pick up.



  • Park Calls by BLF

In this application we can monitor the status of call parking number, park the current call as well as retrieve the parked call.

In the example below, if you are in an active call, you could press the DSS key 1 to park the current call. And press DSS key 1 again to retrieve the call if the parked call is not answered by others.

Or if there is an incoming call already parked on number 6900 by others, we can press the DSS key 1 to answer the parked call.


We have another way to make it happen. With the Yealink phone we choose the Type as “Call Park” and fill in the call park number. For other brands IP phone we are not sure if it works.


Example: A user calls to extension 700,  user 700 answers the call and  presses the DSS key directly to park it to parking number 6900, user 706 use DSS key to answer the parked call.

Step 1. Login the IP phone UI and select the DSS key and choose the type as “BLF” or “call park” type, follow the pictures above to finish the configuration.

Step 2. Same operation to user 706

Step 3. When the call comes, user 700 pick the call and press the DSS key then hang up

Step 4. User 706 hook off and presses the BLF key to answer the call.


  • Seize-Line feature by BLF


With this feature, you could monitor the line status, and seize the line to make an outbound call.

Basically you could press to DDS key configure to "seize" the line. After hearing the dial tone, you could start to dial the number you want.

Step 1. Check the trunk name on Yeastar S-Series VoIP PBX.


Step 2. Configure DSS key on the IP phone, set up the "Value" field with the name of trunk.



1. Only PSTN, GSM trunk are allowed to be monitored in S-Series VoIP PBX as well as have "Seize-Line". But make you sure have configure outbound route the trunks.

2. Please choose the right line in the Line check box, make sure you have chosen the right one  which is register to S-Series PBX.

3. PRI trunk could have "Seize-Line" feature, but no monitoring.



Have more questions? Submit a request


  • 0

    Thanks to Sharon Chan

  • -1

    Why is the seize line feature not working with voip trunks?

    Is there any other possible way to select a specific trunk with a dss key when making outgoing calls?

  • -1

    Hi Pixy.

    I am using the BLF function in S Series with Fanvil X6 phones. The operation with extensions works perfect.

    Also I can monitor FXO line status from the phone, but I am unable to seize the line.  What may be missing? 

  • 0

    Hi there. Please discard my last question, it's fixed. The issue was, the extension did not have permission on the outgoing route.  By the way, monitor/seize external lines  is a great feature , adds value to the solution.  

  • 0

    Hi Gustavo,

    Could you kindly explain to me how you configured the BLF function?

    I'm going crazy....

    I am using an S series pbx and Fanvil X4U phones

    thank you

  • 1

    Hi Stefano,

    please let me know your email address and I will send you a couple of documents.

    Regards, Gustavo

  • 0

    Hi Gustavo,

    my email is:


  • 0

    I'm using X5U and can not get the direct pickup to work on the BLF!! :(

    Please help!!!

Please sign in to leave a comment.