This post discuss the steps to activate Windows Server 2008 Core edition (download link) using a MAK key via the phone. It is much easier to activate via the Internet but for those computers that do not have access to the Internet (for example a Hyper-V virtual machine) you can use the phone. If you have more than one machine to activate you may want to look at the Volume Activation Management Tool 1.1. This supports batch activation for computers not connected to the Internet. This is very useful for isolated lab scenarios.