Colin,
The device (phone) has a CSS and the DN has also a CSS.
The resulting CSS is a concatenation of the Device CSS and Line CSS.
Read the SRND chapter talking about the line/device approach.
The Device CSS should prevent calling PSTN.
The Line CSS should also prevent calling PSTN when configured for your dummy numbers.
For Extension mobility, the User Device Profile should enable calling PSTN.
Therefore, for the same device, if someone is logged out of EM, he won't be able to call PSTN.
And is he is logged in to EM, he will be able to call PSTN.
Pierre.