CUCM 5, 6 and 7 all run the Cisco Voice operating system which is based upon Red Hat Linux. System access is very limited though - there is a CLI running on the server that supports basic commands and a web inteferface that is used for most config, management, backup etc.
I believe that Windows support was on the roadmap for CUCM7 but it was dropped due to the success of the Linux appliance based OS with CUCM 6.
If you are only used to Windows CallManager you are in for a treat as the appliance based approach makes system maintenance far easier - you only need to patch the system once - no more separate patches for the OS, SQL and CCM application. The appliances also have two partitions which allow you to install upgrades to the inactive partition during working hours and then switch versions during an outage.
