One nice feature of Lync (and OCS before that) is the Server’s ability to provide clients with details of errors that occurred whilst the server was attempting to process a SIP command sent from the client. This is done by extending SIP to enable both client and server to report errors to each other. A new SIP header, Diagnostics, is used to report errors The idea is that if the server has an error, the client can get more information and possibly provide better information both to the end user and to any help desk support engineer.
This extension, snappily known as MS-OCER, an hot it is used is defined in the Client Error Reporting Protocol document you can download from Microsoft at: http://download.microsoft.com/download/1/6/F/16F4E321-AA6B-4FA3-8AD3-E94C895A3C97/%5bMS-OCER%5d.pdf
The PDF file, which runs to just under 200 pages, describes The details of the protocol plus full details of every Error ID including the reason for the error, are included in this document. This is a document that is likely to be of great value when you are troubleshooting!
Interestingly, although the document was last updated last November, there appear to be no Lync 2013 specific messages – only those for Lync 2010 and earlier. I assume that the Lync 2013 errors are the same as for Lync 2010, but there’s nothing I can see in the document to confirm that.