To answer #2 I think what you're trying to find out is how would the VM know where to go when the caller calls in for company X. If you have the incomcing calls for company X going to a PhDN (let's just say PhDN 300) and you forward 300 to lets say Mailbox 300 you can set up your greeting in MB 300 to have the options for company X 1 = A, 2 = B, and so on.
All the subscriber mailboxes in a Stratagy can act as call processors. If you have 2 ports dedicated for company Y and 2 ports for company Z it shouldn't cause a problem since company X is being forwarded to VM by a phantom and can hit any of the ports coming in. The problem is with 3 companies and such a small amount of ports on your voicemail you may run into busy ports.
Now if this is 3 companies that don't have a lot of traffic you may be alright but having a recptionist answer the 3 companies first then forwarding what can't be answered to an auto attendant might be a more viable solution.

Just my opinion though. I'm not sure of your situation.

The Rat