I was working on a strange issue at a customer regarding Enterprise Voice from Lync.
The issue:
Some calls fail before call setup completes…In my case it was mostly landline calls, cell calls worked fine.

A wireshark trace showed that Lync was “getting bored” waiting for a response from my SIP gateway. Then sends a CANCEL to which the gateway sends a SIP 487.

The Lync Mediation Server is sending a CANCEL on call setup, after a very short time (seemed like 8 seconds but must have been 10 seconds all up)

The culprit…
After the Lync 2010 CU4 update from Microsoft, Lync has become impatient  and if the remote party has not responded with more that “100 Trying” during 10 sec the Mediation Server sends a CANCEL ! 

This timer was earlier 30 – 40 seconds, but is now only 10 !!
The remote party can’t respond with more that “100 Trying”, until they have received anything from the Called Party.

The fix:

Configuring Parameters
Some of the above timeouts can be configured. The file which has the configurable parameters is ‘OutboundRouting.exe.config”  Use caution when changing these values, as a rule of thumb try not to increase or decrease the value by more than 25% of its original value.
From OutboundRouting.exe.config
       – The culprit
The FailOverTimeout should be increased to the desired time limit.
The file is found under 

C:Program FilesMicrosoftLync Server 2010ServerCore on the Lync 2010 FrontEnd Server.
C:Program FilesMicrosoftLync Server 2013ServerCore on the Lync 2013 FrontEnd Server.

Changing the value from 10000 (10 sec) to 15000 (15 sec) solved the issue.
After changing this value, it’s recommended to reboot the server. I tried restarting services but wasn’t successful.

Next time you run Lync updates this value may be reset to 10000 – Keep a record!
This one caught me out a second time after an update reset the timer to 10 seconds