Whenever an Exchange mailox is enabled for Unified Messaging, a new EUM address is automaticaly created. Valid EUM addresses include the telephone extension or the SIP address of that user:
In order to have the option Call Voice Mail directly from Office Communicator, make sure the SIP URI ([email protected] in this case) is the default EUM address (it should be in bold). You can make it the default EUM address by right clicking the mailbox, going to the E-Mail Addresses tab, selecting the desired EUM address, and then cliscking Set as Reply.