Unassigned Number Treatment

Hi All

Is there anyway to configure how unassigned numbers are treated? 

Ideally i want unassigned numbers for a particular site to route the the auto-receptionist of that site, however this does not seem to be possible. 

Alternatively i cant find any settings or articles on how to add like a audio prompt or anything that will change the current behaviour of the call just dropping in the event a user calls a unassigned number. 

Thoughts anyone??

Many Thanks

Jason