It is most likely Microsoft Lync's configuration problem. See this page:
http://technet.microsoft.com/en-us/library/gg425891.aspx for ports to be opened and forwarded. Also read this thread:
http://social.technet.microsoft.com/Forums/en-US/ocsedge/thread/aeaf89a1-e8ab-44d8-9183-6e430e506168/ for any hint that can lead to correct Lync configuration.