Call loop between Lync and Gateway


As part of ongoing expansion we recently added another DDI range. Most of the new range would be dormant initially. Of course I wanted to catch all the calls that were unassigned and send them off to a RGS.

Here is where the plot thickens…
This seemed to work well in Lync 2010, now I find myself on 2013 and see a few interesting new features…like trunk-to-trunk routing.

So I ring the unassigned number and low and behold, the gateway sends the call to Lync. Lync sees the number as unassigned and does a trunk-to-trunk transfer sending the call back to the gateway. You know the rest of it right, the gateway sends the call to the Telco who sends the call back to the gateway and then back to Lync and before we know it all the SIP channels between Lync and the gateway are saturated.

The caller simply hears what seems to be a longer than usual set up time and then the call fails.

So by now I am thinking why isnt my unassigned number config working…
A similar issue on Technet leads me to believe that the issue is resolved in Lync 2013 July here 

Not in my case though…

Further down the same Technet page I came across a reference to Chris Norman’s article on Call Park retrieval Issues here (nice article Chris!)

I also came across this blog http://d1it.wordpress.com/2013/05/14/pstn-to-lync-2013-unassigned-number/ (again nice work MLamontagne)

Turns out the gist of the matter is that the trunk-to-trunk transfer is engaging prior to the call reaching the Unassigned number config.


The SolutionAs per Chris’ findings, removing all the PSTN Usage records from the Trunk configuration which in effect prevents the Trunk-to-Trunk transfer from engaging.

If you needed Trunk-to-trunk routing a separate route should be built for the purpose.

Removed the PSTN usages from the route, and voila!

Microsoft has apparently addressed this issue in the Lync 2013 July Cumulative Update, I can confirm that this update DID NOT do it for me.

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About Paul B

My name is Paul Bloem and I am employed at Lexel Systems in New Zealand as a Principal Consultant for Unified Communications. I have been working on enterprise voice solutions for over 20 years. My first 10 years were spent working for a Telco in South Africa (Telcom SA). This is where all the groundwork happened as I was exposed to just about every aspect of telecommunication you could imagine. I develop an interest in PBX technologies and eventually became the go-to guy. Next, I had a 10 year run at Siemens South Africa, most of my time there was as a Technical Trainer. During this time VoIP hit the world stage, I had the privilege of introducing VoIP both as H.323 and later SIP across the Siemens HiPath 4000 solution stack. In 2008 I immigrated to New Zealand with my newly attained MCSE, I was ready to go where no PBX Techie had gone before. I was employed to explore OCS 2007 and that was pretty much the beginning of the end for me. I have been working on OCS and Lync ever since. My current role focuses exclusively on Lync and associated technologies.. That includes pre-sales, consulting, architecture and design, training and support. I even get to play in the development space from time to time - focus on play ;-) I was nominated as a Microsoft VTSP for Lync early in 2013 and also awarded Microsoft's MVP award for Lync in 2014.
This entry was posted in 403 forbidden, 404 not found, 486 Busy Here, Call loop between Lync and Gateway, Lync, Lync 2013, SIP 403, SIP 486, SIP 487, SIP Channels get Saturated, Unassigned Number. Bookmark the permalink.

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