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:
Rejoin the meeting for the conference organizer in Lync 2013 (Skype for Business).