You should be aware that WM 5.1 is not a supported client with OCS in general:
The following clients are supported for users who are homed on Office Communications Server 2007:
· Microsoft Office Communicator 2005 only for IM and legacy presence
· Microsoft Office Communicator 2007
· Office Live Meeting 2007 client for Web conferencing
· Conferencing Add-In for Microsoft Outlook 2002, 2003, and 2007
· Microsoft Office Communicator Web Access (2005 release) only for IM and legacy presence
· Microsoft Office Communicator Web Access (2007 release)
Windows Messenger 5.1 and Communicator Mobile (2005 release) are not supported by Office Communications Server 2007.