Here are the steps you need to follow:
1.Open Outlook.
2.Create an online meeting.
3.Select “Meeting Options”.
4.Select “Audio and Phone” from the left menu.
5.Tick off “Turn on entry and exit announcement for this meeting” (by default, this box is unchecked).
6.To make this your default setting for all online meetings, tick off the box labelled “Remember Settings” at the bottom-left corner of the window.
7.Once completed, select ‘OK’ and you’re done.
http://blog.navantis.com/how-to-turn-on-entry-and-exit-announcements-for-microsoft-lync/