Outlook Web App now supports the use of SIP client integration with Office Communications Server. This provides a great feature set for users, but there are some things you must do when integrating with Communications Server to make it all work. Microsoft Exchange Server 2010 offers a new enhancement to its Client Access server role called Outlook Web App. The new Outlook Web App feature is very similar to Microsoft Office Outlook Web Access. Besides the name change, Outlook Web App offers a new feature that enhances the remote user’s capabilities for inter-client communications. Outlook Web App now supports the use of SIP client integration with Office Communications Server.
More Info: TechNet