In case you need it, here's some more info:
Client A is running Windows 2000, SP4, with MOC 2007 (R1) with the July 2009 update pack
Client B is running XP SP2, MOC 2007 R2 with the June 2009 update pack
The OCS Front End is EE,2007 R2
When client A calls client B or vice versa using voice or video, both clients display an "incompatible security" error when the call is answered