Sometimes you might like to modify the way that SIP messages are processed on Lync servers. Microsoft SIP Processing Language (MSPL) allows you to do just that. You can write a MSPL "message filter" to filter and route/reroute messages in Lync Server 2010 applications.
(HT: Ken Lasko for a really good example of using this feature to route calls that would fail without it)
What kinds of things can you do to a SIP message with MSPL? A full list of MSPL built-in functions can be found here, but just some examples are:
AddHeader – Adds a header to the current SIP message
Fork – Creates a forked copy of the current message with its request URI field set to the supplied URI string.
Respond – Generates a SIP response to the current request message with the supplied status code and reason phrase
Proxy Request/Response – Proxies the current SIP request or response.
Check out the additional references below.
MSPL Scripting Reference: http://msdn.microsoft.com/en-us/library/gg425301.aspx