In case you try to activate a Windows Server 2012 R2 x64 evaluation version, you can not enter the product key of any valid Windows Server 2012 edition. The license manager is explaining: “That key can’t be used to activate this edition of Windows. Please try a different key”. A general error will be displayed if you are using the slmgr -ipk  command.

Convert the Evaluation Version to your desired Edition

Open DOS command prompt and enter following commands:

# dism /online /Get-CurrentEdition
# dism /online /Get-TargetEditions
# dism /online /Set-Edition:StandardServer /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula

Now you should be able to activate Windows Server 2012 R2 x64 Standard

Offline activation

In case you have to do an offline activation by using your phone, you can launch the activation wizard by using following command in your DOS prompt:

# slui 4

For additional ressources please have a look at the related Microsoft TechNet article:
https://msdn.microsoft.com/de-de/library/jj574204(v=ws.11).aspx