CallManager 5.0 was released in parallel with CallManager 4.2 on 6 March 2006. Cisco Unified CallManager 5.0 is Linux based (Cisco VOS)
Version 6 was supposed to merge all features of the Linux appliance (SIP support and licensing requirements) between the Linux platform and Windows version, and it was based on Red Hat Enterprise Linux. The released version of Unified Communications Manager will not support the Windows platform. This version will install natively on VMWare for lab purposes.
Version 7 was originally stated to be available in both Windows and Appliance models. However Cisco has since stated that OS independence.
CUCM 8.X supports virtualization.
More info on CUCM data sheet.