The only way you can do this PRE BES 5 SP3 is to use an OCS 2007 RTM CWA server (we're running in a similar situation) The only way to do so natively with OCS 2007 R2 is via SP3 and installing all of the OCS components on your BES.
Requirements from the following RIM
link
"To access Microsoft Office Communications Server 2007 R2 or Microsoft Lync Server 2010 on your BlackBerry smartphone, you will need:
BlackBerry Client for use with Microsoft Office Communications Server 2007 R2 and Microsoft Lync Server 2010. Download BlackBerry Client for use with Microsoft Office Communications Server 2007 R2 and Microsoft Lync Server 2010
BlackBerry Enterprise Server v5.0 with Service Pack 3. Download BlackBerry Enterprise Server v5.0 with Service Pack 3
BlackBerry Device Software v4.5 or higher"
Before proceeding further I would review ALL of the BES OCS integration/setup documents and make sure you're setup as per RIM's requirements. If you still have probs I would engage RIM support.
Regards,
M@