Microsoft Lync Web App can be used to join Lync meetings from a web browser on Windows and Mac operating systems. One of the first steps most users would perform after joining a meeting is to connect to meeting audio. Lync Web App supports users joining meeting audio via a PSTN phone, or via a previous version of Communicator like Communicator 2007 R2.
The following screenshots illustrate the steps a user joining a Lync meeting as Guest (unauthenticated) via the Lync Web App has to take to join the meeting audio.
Read more at source: http://communicatorteam.com/archive/2010/10/26/1906.aspx