How to Use SLA


Shared Line Appearance (SLA) feature helps users share SIP trunks and FXO trunks. It also helps monitor the status of the shared line. SLA feature works with BLF key on IP phones.

  • When an incoming call is received, all the SLA stations are informed of it and may join it if the shared line allows to barge in.
  • When an outgoing call is made by one SLA station, all members shared with the same line are informed about the call, and will be blocked from this line appearance until the line goes back to idle or the call is put on hold.

To use SLA, you need do the following:

  • Enable SLA feature on a FXO trunk or VoIP trunk.
  • Create SLA Stations.
  • Configure BLF keys for the shared line on the stations' IP phones. The BLF key value is "extension number_trunkname". 

SLA Sample Configuration

You need at least two IP phones to set up SLA. In a boss-assistant scenario, the boss will share the FXO trunk 5503305 with the assistant. If the boss’s phone is extension 5000 and the assistant's phone is 1000, then you can set up a shared line by following these steps:

  1. Go toSettings > PBX > Trunks, edit the FXO trunk, check the option Enable SLA and configure SLA options.
    • Allow Barge: check this option, the other SLA station shared the same line could press the BLF key to join the conversation.
    • Hold Access: if set to open, other stations that share the line could retrieve the call; if set to private, the call can be retrieved only by the station that previously put the call on hold, not by others sharing the same line.
  1. Go to Settings > PBX Call Control > SLA to create SLA stations to monitor and share the FXO trunk 5503305. Here we create two stations, one for extension 5000, one for extension 1000. 
  1. Configure BLF key on the boss's IP phone(extension 5000). If the configuration is correct, you can see the BLF key LED is green.
  1. Configure BLF key on the assistant's IP phone(extension 1000). If the configuration is correct, you can see the BLF key LED is green.
  1. All the configurations are done. You can enjoy the SLA feature now.

SLA Operations

  • Making Outgoing Calls
    If the BLF key shows the shared line is idle, the boss or the assistant could press the BLF key to use the line. After hearing a dial tone, he/she could dial the external number to call out.

    If the BLF key LED turns red, which indicates the line is in use now. The other SLA station could not use the line to make outgoing call now.
  • Handle Incoming Calls
    When an external call reaches the shared line, all the SLA stations will ring. Any of the stations could pick up the call. 
  • Bargining-in an Active Call
    When there is an active call between an SLA station and an external number through the shared line, other SLA stations could join the call by pressing the BLF key if “Allow Barge” is enabled for the shared line.
  • Hold and Retrieve Calls
    During the call, the SLA station could press the BLF key to hold the call.
    If Hold Access is set to "open", other stations that share the line could retrieve the call;
    If Hold Access is set to "private", the call can be retrieved only by the station that previously put the call on hold.


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  • 0

    I have used the exact configuration and when the SLA trunk is called it doesn't ring on the SLA extension.

  • 0

    Hello Sir,

    1. Which version of firmware you tested with?

    2. Which kind of trunk you test with?

  • 0
    Avatar and fxo trunk

  • 0

    Hi Sir,

    I tested this with one of the demo S100 in our office with the same firmware version and FXO trunk, the incoming calls works fine.

    Please check:

    1. SLA was configured properly on S Series PBX, on both trunk page and the SLA page;

    2. BLF was configure properly on the IP phone, pay attention to the "value" and "Line" settings of the DSS key, you need to change according to your setup;

    3. If you setup properly, the LED light of the BLF key will turn Green, if not turn Green then you need to check the configurations. 

  • 0

    As I said before, I have it configured correctly but it doesn't work. Trunk name is voip.ms3 and extension is 2101 BLF key (2101_voip.ms3) Line 1 only active account..

    1. BLF is not green

    2. Press key to make call/ get dial tone/ dial number/ display shows talking but call times out

    3. When calling SLA trunk the call rings to extension 2100


  • 0

    Hello Sir,

    As testing in our office, the BLF will turn Green once finish setup. The incoming calls works fine, the SLA stations ring as expected, we can use the BLF key to answer and hold the call. While outoing call, we do have a problem.

    Regarding your test results, the BLF key is not turning Green, please check below:

    1. SIP turnk is registered correctly or not; You can check the SIP trunk status on PBX monitor page;

    2. If SIP trunk status is fine, please create a support request ticket and ask for remote assistance if needed.

  • 0


    Hello Gary, I worked with Abby from your Yeastar support team at 4 am EST for two hours this morning. She was able to access my system but had a great deal of difficulty isolating the problem. Eventually she determined that the issue was related to a few bugs that needed to be worked out in the software. She custom configured some extensions in asterisk and now all inbound calls are routed to their correct destinations. As for the sla trunk and the sla extension, the calls are going to the sla extension but I cannot access the sla trunk from the sla extension nor does the blf turn green. What I don't understand is that you say it works perfectly fine on your system with the same software I have and she says it is a bug in the software.. Why is that and why haven't this been tested more thoroughly? The system should be able to perform as advertised!!!!! I should not have multiple issues with trunk routing, inbound, outbound, blf keys and sla applications!!!


    Edited by Alton
  • 0

    Hello AI,

    We do have some bugs, I appology for the issues/bugs we have. Sorry for my answers post here makes you confused.

    Why we get different results is caused by one of the bug affect the SLA incoming calls happen only under specific environment.

    We have confirm the bug, which happen with some specific SIP trunk providers.

    When register multiple SIP trunks to the same SIP server, if we setup different incoming destination for these SIP trunks, only one of the inbound destination will work, this affect the inbound destination of SLA as well. That is the reason why the incoming call doesn't ring your SLA station.

    For the SLA outgoing call, we do have issue on version This is confirm as another bug.

  • 0


    I am trying to set up an SLA on a Yealink T27G IP phone. When I press the button the SLA is assigned to I get a recording that says please dial the number you want to call and then I get a dial tone but the SLA line is conferencing with my main line and IDs with the number for the main line on the outgoing call. I thought when I pressed the SLA key I would select and dial out on that line.

  • 0


    Your thought is correct, when you press the SLA key you should select and dial out through that line, if not, then the SLA doesn't function properly, I would suggest you to submit a ticket to follow your issue.

  • 0

    Hello Support,

    I have them working but there's a voice announcement when I press the line that says "Please enter the number you want to dial" then will here the analog dial-tone. Is there a way to get rid of the voice at all?

    Thanks for your time and hope to hear back from you soon.

    Kind regards,


    NOTE: Found the solution and have disabled it now

    Additional Query: Now, how do I route the calls to an extension vmail when I'm using SLA?

    Edited by Alvin
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