Lync cannot verify that the server is trusted for your sign-in address. Connect anyway?
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.

In the most environments, the SIP domain is different from the Active Directory domain.


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