Windows Server 2008 R2 Enterprise Server License
Windows Server 2008 R2 Enterprise is an advanced server platform that provides more cost-effective and reliable support for mission-critical workloads. It offers innovative features for virtualization, power savings, and manageability and helps make it easier for mobile workers to access company resources.

Hyper-V
Windows Server Hyper-V, the next-generation hypervisor-based server virtualization technology, allows you to make the best use of your server hardware investments by consolidating multiple server roles as separate virtual machines (VMs) running on a single physical machine. With Hyper-V you can also efficiently run multiple different operating systems—Windows, Linux, and others—in parallel, on a single server, and fully leverage the power of x64 computing. Hyper-V provides a dynamic, reliable, and scalable virtualization platform combined with a single set of integrated management tools to manage both physical and virtual resources, enabling you to create an agile and dynamic data center.

Licensing
A Server License is required for each operating system environment running Office Communications Server 2007. A User/Device CAL is required when a user or device is directly or indirectly accessing a Windows server. Additionally, if the user or device is accessing or using the Remote Desktop Services functionality of Windows Server, a Remote Desktop Services CAL is also required. A minimum of 5 CALs is required.