Restrict extension-only CAP to internal calls only

I'm creating a CAP using a Common Area Phone package and only assigning an extension. There is no phone number or DID assigned. There is a desk phone needed - Poly Edge E220. Is there a way to restrict this phone from making external calls? I've created speed dial buttons for the key internal numbers it needs to reach. I've disabled international calling. But I can't find a setting to stop it dialing other external calls that will be metered. Am I missing something? 

Thanks,

Chris