OCS 2007 comes with 2 types of Cals. The standard CAL provides IM and File Tranfer. The Enterprise CAL provides Telephony and Conferencing. If you purchase the Enterprise CAL, you also need to purchase the standard CAL.
Now comes the Edge Server licensing for external connections. The Standard and Enterprise Licenses are the same price and have to be purchased only once. These licenses allow for the external and/or NON-staff connections.