We recently came across an issue where a number intermittently failed to dial and we needed to find out where the call was falling over. We used the Lync Server Logging Tool to troubleshoot the issue, which can be found in the the Lync Resource Kit Download.
Once installed, open the tool from the Start Menu:
Ensure SIPStack, S4, UserServices and All Flags are selected for each:
Select “Start Logging” then reproduce the problem you are having. Select “Stop Logging” and hit “Analyze Log Files”:
Select the “Messages” and you should see the error in red:
This should point you into the right direction to fix the issue or put the error message into a search engine for online help.