What kind of OCS clients do access the EWS services? In this post of the Communicator team there is a lot information about that.
In the Unified Communications (UC) world 2 types of clients access Exchange 2007 Web Services (EWS) running in Client Access server role (CAS). These two clients are Office Communicator 2007 (OC) and Office Communicator 2007 Phone Edition (OCPE). The Office Live Meeting client, Communicator Web Access and Office Communicator Mobile do not access EWS.
OC uses EWS only if the user is running Outlook 2007 on the desktop with mailbox running on Exchange 2007. OC uses EWS only to access Out Of Facility (OOF) Note and Calendar data. More information about OC can be found here.
OCPE uses AutoDiscover and EWS on the CAS servers. It uses AutoDiscover to locate EWS. OCPE refreshes AutoDiscover information every 24 hours. EWS is used to fetch Outlook Contacts, Voice Mails and Call Logs for the signed in user. OCPE uses EWS only if the signed-in user is enabled for Enterprise Voice and Exchange 2007 UM. If the user is not enabled for Exchange 2007 UM OCPE will not show the tabs for Voice Mails and Call logs.
Read more at source: http://communicatorteam.com/archive/2008/03/18/130.aspx