Exchange Server 2010 UM provides OCS 2007 R2 with the voice mail feature, and OCS 2007 R2 can make presence information and instant messaging features available to your OWA users. You can also configure an automatic switchboard for your OCS 2007 R2 voice-enabled users using an UM auto attendant.
UM can also utilize an existing IP PBX that is configured with OCS 2007 R2—you do not need additional hardware to connect UM to your PBX if OCS 2007 R2 is installed already. Thus any PBX configuration can be managed from the OCS 2007 R2 side, and does not need to be configured again in Exchange 2010.
OCS 2007 R2 also provides other features that integrate into UM:
Instant messaging The OCS 2007 R2 client provides ...