In a recent deployment I had a customer that wanted to enable the feature of allowing external parties into a Lync conference however they did not want the internal users to be able to connect from a Lync client externally ie not on corporate approved devices.
When we sent out the meeting invites and external users tried to join they received the below error message.
We looked at the Federation and External Access Configuration and with some troubleshooting and testing we found that we needed to turn on Remote User Access on both the External Access Policy and Access Edge Configuration tabs of the Global Policy and Configuration the above error was resolved.
Here we have enabled Remote User Access on both the External Access Policy and Access Edge Configuration tabs of the Global Policies and Configuration as well as created a Site External Policy with Remote User Access disabled.