I did a bit of digging into it and think I’ve worked a few things out that make the whole situation a bit clearer. We need to look at how often Lync polls Exchange for the free/busy information, and then how often your calendar state is published to your Lync presence i.e.you change from Available to In a Meeting based on the free/busy data that was retrieved from Exchange.
Fortunately, there is a well respected work-around, create a new Global Address List, and then 'update' the mailbox. Here is a free tool to monitor your Exchange Server.
Download and install the utility, then inspect your mail queues, monitor the Exchange server's memory, confirm there is enough disk space and check the CPU utilization. Solar Winds provides this fully-functioning freebie, as part of their commitment to supporting the network management community.
Free Download of Solar Winds Exchange Monitor As an alternative to clicking in the Exchange Management CONSOLE, you could type commands in the Exchange Management SHELL.
Make sure Domino is configured as a service to ensure there are no problems with Free/Busy Coexistence.
In our migration, we chose to use the Disparate Namespace Model.