There are three steps:
- Add the following key to the registry: HKEY_LOCAL_MACHINE\Software\Microsoft\Office\Outlook\Call Integration
- Add a dword-value: TAPIOVERRIDE and set it to 1
- Restart Outlook.
In addition you can customize the ribbon to have the Call-button directly on first sight.
Just right click the start(contacts) ribbon and say Customize the ribbon. Add a new group to the Start(contacts) ribbon rename it (i used TAPI) and add the Call-Command to that group. A possible position for the new group is after the Delete group. Worked fine for me