Lync 2013 provides a single, unified client for real-time communications, including voice and video calls, Lync Meetings, presence, instant messaging, and persistent chat. The Lync client has a fall back option so that, in the event the Lync client cannot make a secure SIP connection to the Lync server, it will fall back to an unencrypted HTTP connection. In that event, all traffic will be unencrypted and in clear text. The configuration must be set to prevent HTTP being used for SIP connections in the event TLS or TCP fail.
Set the policy value for Computer Configuration -> Administrative Templates -> Microsoft Lync 2013 -> Microsoft Lync Feature Policies 'Disable HTTP fallback for SIP connection' to 'Enabled'.
The following resource is also helpful.
This security hardening control applies to the following category of controls within NIST 800-53: System and Communications Protection.This control applies to the following type of system Windows.