Problem
Lync cannot verify that the server is trusted for your sign-in address. Connect anyway?Cause
Lync Client 2013 has an additional safety check implemented in that the users SIP Domain  is compared with the FQDN of Lync server when the user tries to connect.

here you need to modify or add the “new String Value” TrustModelData
in this key, you need to add the server listed in the warning.
e.g. lyncpool.lynclab.local