Hi Papa,
I have worked a little on this integration and your situation sounds kind of familar. There are a couple of things you may want to check. The first is the version of VCS. I believe 4.2 may work better with R2 as it does do some fixup of the SIP packets as some tanberg endpoint dont know how to deal with. Also you will need to set the encryption setting for the pool or locally on your PC. Tanberg cant do encrypted media with OCS. Its something Tandberg is working on.
Below is the local PC setting you can use to change the encrypted media from required to allow in Communicator R2.
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Communicator]
"PC2PCAVEncryption"=dword:00000000
Hope this helps.
Cheers
Chris
http://voipnorm.blogspot.com/