In a Lync Server 2013 (Skype for Business) environment, you set the AllowIPAudio parameter toTRUE in the conference policy settings. When you invite a third user to a PC-to-PC only instant message (IM) conversation, the peer-to-peer session is promoted to a conference. In this scenario, the meeting organizer can't make an audio call to attendees. Here's the screen shot for the unavailable Call button:
To fix this issue, install the November 10, 2015, security update (KB3101496) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.
To work around this issue, use one of the following methods:
Set the AllowIPVideo parameter to TRUE on the organizer's conference policy.
Enable Enterprise Voice for organizers.
Rejoin the meeting for the conference organizer in Lync 2013 (Skype for Business).