While the functionality you state is "true". User and Device mode is not supported. FreePBX generally assumes you are running in Extension mode and therefore combines lots of the functionality therein in other modules. When you are in User and Device mode there is a separation of "two" in Extension mode the two are combined into one and thus other modules make this assumption as well.
The issue you've run into here is a prime example of the unsupported nature of User and Device mode.
So sorry, it's not supported.