Join button missing on Lync Meetings from federated partners

You might have the situation that you are receiving Lync Meeting invites from federated partners using Exchange 2013 and Lync 2013, but when Outlook 2013 or OWA 2013 CU1 brings up the reminder dialog for the meeting, the Join button is missing. Outlook and OWA will show the Join button based on the existence of…