asterisk users - Aug 2005 - way to prevent voicemail dialout/callback from 'outside'

I am trying to find a way to allow dialout from voicemail when connected
from an 'internal' extension context, but prevent dialout when connected
from an 'external' extension context.

As far as I can tell the dialout context that can be set in voicemail
has no regard for the context from which the call to voicemail came in.

Any ideas on this? Maybe a variable passed when voicemail is called?