We've identified a number of tenants that are using older clients with a bug that's causing media sessions to fail to establish. If you were to diganose these issues, you would see in the logs a possible report of “TurnAuthUnknownUser” . We have created a report you can use by uploading it to https://cqd.lync.com to identifiy the number of different clients in use in your environment. You can thereby work to update your users to a recent and consistent version.
To see if you are having this issue, download the attachment to this article and upload it into https://cqd.lync.com by entering the detailed reports page and chosing "Import" from the left hand menu.
Once imported, a very important change must be made to ensure only clients in use in your own deployment are being checked and you aren't reporting on federated or meetings being joined from federated partners hosted on your tenant are being shown.
To do that, go to the new report you have just imported. By default it will be all the way to the right, the last one in the list of detailed reports.
On that report, hit the "Edit" link as pictured above.
Once within the Edit interface for the report see the "Filters" section at the bottpom of the left pane.
Within the Filters; select "Second Tenant ID" and paste in your tenanat ID.
To retreive your tenant ID, the simplest way is to sign into your Skype for Business Admin portal and see it on the "Dashboard" tab you are on when you first access the portal. To access the portal sign into https://portal.office.com/admin/default.aspx and use the service admin portals section on the bottom left and select "Skype for Business" to be directed to your forest and tenant admin page as pictured. TenantID is shown on the admin portal as "Organization ID". The GUID represented there should be pasted into your CQD report.
Please refer to Office 365 client update channel releases article to see how latest client version available at https://technet.microsoft.com/en-us/office/mt465751.aspx.
As additional information, please see the Change Management for Office 365 Clients article below to help understand what the update methodology is and how you can manage and allow for automatic updates while still maintianing a level of control within your organization https://technet.microsoft.com/library/mt584223.