This issue occurs when you apply a static route in a Microsoft Lync Server 2013 environment.
This issue occurs because the in-memory-only route information is lost on a Lync Server 2013 Front End server.
To fix this issue, install the January 2017 cumulative update 5.0.8308.984 for Lync Server 2013, Front End Server and Edge Server.
The issue should typically occur during service startup. The best workaround to use until the January cumulative update can be installed, is to test the static route right after the startup of the service and restart the service if the static route doesn't work.