OCS doesn't support fax however you can use Exchange 2007 Unified Messaging which does support inbound fax and will deliver them to a users Outlook Inbox.
UM 2007 also acts as the 'voicemail' for OCS users of course.
You can also use third party LAN fax servers that support FoIP (T.38) and arrange for the PSTN gateway to deliver fax calls directly to the fax server and then to the users Inbox. This will generally give you send and receive fax.