Verify 

Microsoft Lync Server

 2013 settings

To activate the Lync plug-in, you need to have enough ports open between the 

NDS

 server and the end users. Because each Lync call requires two ports and there is a lot of overhead during call initiation, you might not be able to make more than 5 calls with 40 ports. It is recommended to use at least 1000 ports, although testing 45 concurrent calls with 200 ports was successful. This number of ports should also be enough for additional concurrent lines.

Two settings control the configuration of the ports: the Lync policy and the network settings.