After applying Cumulative Update 3 for Lync (KB 2571546), users were unable to place or receive phone calls. In Lync, the error was ID 500 (source ID 239):

err500

The SIP Trace on the server showed:

Start-Line: SIP/2.0 500 Server Internal Error

ms-diagnostics: 5016;reason="Request handler threw an exception";source="(server).catapultsystems.com";appname="http%3A%2F%2Fwww.microsoft.com%2FLCS%2FOutboundRouting"

I could not find any helpful information on the internet.

Resolution

Reboot the server after applying the update. The update does not prompt for a reboot, but after rebooting the server, the error goes away.

This may be isolated to installing CU3 on Survivable Branch Servers (SBS) because I did not experience this when applying CU3 on a dedicated Mediation Server.

Hopefully this blog will be indexed by the search engines and be helpful to someone frantically searching for this error! =)