Detecting busy DAHDI trunks with Snom or Linksys

Hello Asterisk experts,

In legacy PBXs, such as Panasonic, if the system has 3 trunks, there are 3 leds in the telephone, each led changes to red when the related trunk is busy by some user. It also changes to green when trunk is busy by this user.

Would you please advice how to setup such feature in Asterisk with Snom 870, Snom 370 or Linksys SPA942 to detect where some DAHDI channel trunk is busy?

What you are asking for is Shared Line Appearance. I looked into this for a customer over two years ago and got it working, but it did not work well. The customer ended up just letting me setup a group and they just parked calls plus used the buttons on their phones to see when people were on the phone. As far as I remember shared line appearance is/was a hack and not really recomended. I dont know if thats changed I have never needed it since then, but here ya go

Actually it is presence, not SLA.

IP telephony doesn’t have indentifiable trunk circuits, only trunk groups.

Circuit switched connections (dahdi/zaptel) are normally addressed as trunk groups as well. I’m not sure whether this can be broken down for presence purposes, but if it can, you need to search for “dialplan hints”.

If you really need this feature and it isn’t possible to break down a trunk group for presence purposes, you may have to do the hunt for a free circuit explicitly, in the dialplan.

Presence can only be output to IP phones.

Actually, you could simulate a key and lamp system by using SLA, but I can’t think why you would want to, when it is trivial to have the system hunt for a free circuit. You would basically associate each trunk with an extension number, then make each phone share all those extension numbers. The normal modern use of SLA is for a secretary to share all the extension numbers of the people they work for.

Thank you riddlebox and david55,

Actually I am moving many businesses from legacy Panasonic systems to Asterisk. Most of them have few analog FXO trunks (three or four), and they feel more control when they see which FXOs are busy now as they used to. I am receiving complains that they are losing control and they don’t know what’s happening and why lines are busy. But with SLA (or presence) with BLF, the can expect what’s happening in their office. I hope digium will consider such needs in future releases. Also it will be great to allow some users to whisper, spy, or pick up some channel by the same way (as many legacy systems actually do).

take a look at

If they want to see what DAHDI channels or SIP phones are in use this will tell you. Also allows you to listen in, pick up calls etc.