During the transition to Exchange 2013 from Exchange 2007, Exchange Web Services (EWS) integration for Lync will be unavailable for users whose mailboxes remain on Exchange 2007.
The Bad News
Jeremy Silber has an excellent blog article on this issue. It bears reading if you fit the above scenario. Jeremy shows in extremely clear detail what is going on, and why. Unfortunately, he also shows that there is no fix. Here is the article: