Lync Server 2013, Response Group Service has declined the call. Request coming from '%1' was declined because user '%2' sent ...

Lync Server 2013, Response Group Service has declined the call.

Request coming from '%1' was declined because user '%2' sent an INVITE for a conference.
Cause: The client might be unsupported or not valid.
Resolution:
Check the Front End events and the version of the clients.
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