If Unified Messaging (UM) is enabled for your mailbox, you can obtain the UM Access Number and extension in the following way:
Go to Exchange Mailbox > Unified Messaging, in the Access properties area, the Telephone Access Number and Extension number are displayed.
Note: When Unified Messaging is enabled for your mailbox, see the "Welcome to Exchange Unified Messaging" e-mail message to obtain UM Access Number, extension, and PIN.