Lync Inter Trunk Routing causing loop

Problem description

All out of sudden, I noticed that Lync 2013 was looping PSTN calls to unattended numbers out to the SIP trunk. This was causing a loop situation when the SIP trunk provider returned the call to Lync.

This was causing a high Audio Failure Trend in the Lync Monitor reports:

The details found in the Lync Monitor reports was specific about the loop detection:

Gateway responded with 482 Loop Detected

Wireshark on the Mediation Server would also pinpoint this loop:

Several loops before call rejected

Loop Detected by SIP provider 

Solution

After going through all the obvious troubleshooting in order to address this problem, I tried to implement the Lync Announcements for Unassigned Numbers. This was working Ok when calling the unassigned number from a Lync client, but incoming PSTN calls to an unassigned number did still lead to the Loop Detected error!

I came across D(one) IT's article named PSTN to Lync 2013 Unassigned Number Failure. This article made me create a Lync Trunk with no PSTN usage records:

 



After syncing the Lync Topology I noticed that the Lync Announcement for Unassigned numbers was working as expected on incomming PSTN calls:

When this was working ok, I did disable the Lync Announcements for Unassigned Numbers. This stated that the loop situation also got fixed by the Lync Trunk With no PSTN usage records. Lync is now rejecting the call without re-routing it:

 

"Inter-Trunk Routing" in Lync 2013 is an exciting new feature in Lync 2013 that allows Lync Server to be the central call processor for a multi-PBX deployment, but it did cause some problems in this topology with a single Lync topology connected to PSTN through a single SIP Trunk. More information about Inter-Trunk Routing can be found at the following resources:

• Mastering Lync: Inter Trunk Routing Deep Dive
• UCKen: Inter-Trunk Routing in Lync 2013
• What's New in Microsoft Lync Server 2013 : Voice Enhancements - Inter-Trunk Routing