This issue occurs constantly on some Microsoft Lync Phone Edition telephones that connect to Microsoft Lync Server 2010 or Lync Server 2013, even though the telephones are authenticated by using the personal identification number (PIN).
Cause
This issue occurs because the primary server that the Lync Phone Edition telephone registers with restarts. Then, Lync Phone Edition signs in to another server that the telephone is not authenticated.
Resolution
To resolve this issue, install one of the following cumulative updates, as appropriate for the telephone that you use: