You have a Lync Server 2013 infrastructure that is used for conferencing and Enterprise Voice.
Only users who are enabled for Enterprise Voice can use the callback feature. Other users report that they cannot use the callback feature.
You need to ensure that all of the users who are NOT enabled for Enterprise Voice can use the callback feature.
What should you use from the Microsoft Lync Server 2013 Control Panel?
To answer, select the appropriate object in the answer area.
Hot Area:
Answer: 
Explanation:
Note: Configure callback through Conferencing