Check out the latest Nov 2009 updates released for the different Office Communications Server 2007 R2 server roles as described in KB968802. This applies to both the Standard and Enterprise Editions.
A very important and welcome addition to assist the patch management process is the “Cumulative Server Update Installer” (ServerUpdateInstaller.exe) delivered as part of this release. Instead of having to determine and manually applying the relevant patches to the various R2 server roles, this tool relieves the administrator from those tedious chores by applying all updates for the appropriate server role in just one click. You can also use this tool on the command line with the switches /silent, /forcereboot and /extractall.
If not already present, you shoud also apply the update* for the Office Communications Server 2007 R2 Back-end Database (KB969834).
Download for the updates (.msp), executable (.exe) and installer (.msi) can be obtained here.
Here is a good guide on how to install the updates: http://blogs.technet.com/ucspotting/archive/2009/11/26/3296447.aspx