This issue occurs after you click I will dial in to the meeting in the Join Meeting Audio dialog box, and then you dial in to the meeting as an authenticated public switched telephone network (PSTN) user in Microsoft Lync Web App in a Lync Server 2013 environment.
When this issue occurs, your audio modality icon in the meeting roaster is still gray after you join the meeting. Additionally, when you rest the mouse pointer over the audio icon, the tool tip shows the "Not in the call" message. However, your audio modality icon is blue in other participants' clients.
To resolve this issue, install the May 2015 cumulative update for a Lync Server 2013 web components server.