Chuck,
So if I am understanding this correctly the site that is providing the message, "You have reached an invalid extension", is the one that doesn't have the Mailboxes, but it does have some sort of Call Routing Announcement or Auto Attendant and someone is trying to dial an extension from it; is that correct?
If that is the case then you most likely just need to create an Extension ID on the system without mailboxes for those that do not have them and it should solve your problem.
Thanks,
TE