Lync Server 2010, Response Group Service has declined the call. Request coming from '%1' was declined because user '%2' sent ...

Lync Server 2010, 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.