Consider the following scenario:
You deploy four front-end servers and a survivable branch server (Windows SBS) in a Microsoft Lync Server 2013 enterprise standard pool.
You register multiple Lync phones to the SBS.
Many users sign in to all available Lync phones.
The SBS is then shut down.
In this scenario, the Lync Phone Edition devices may be unable to sign in to the front-end servers automatically. Additionally, the users may receive a 404 error code.
To resolve this issue, install the following update:
2937305 August 2014 Cumulative Update 5.0.8308.738 for Lync Server 2013, core components