How about leaving the main hunt numbers on analog CO ports and use a 4 port DID card for the private numbers:

Assuming you call one particular private number several times in a row. The first call is answered by the user, then you could only get 3 more callers through to voice mail? Right? Because that ties up all 4 of the DID ports on the card?

So what happens when a call comes in to one of the other private numbers during the time when all 4 are busy? Busy, right?

Hammer