Unity routes the call to the mailbox based on forwarding number or the calling number. If Unity sees yyyy number, it's going to match one unique subscriber. In unity itself, the numbers have to be unique so you will have to do some sort of number manipulation before presenting the digits to unity. The other option might be to put a unity server in the remote site, and have those phones dial a different pilot point.
Someone else may have an idea how to manipulate the numbers but in Unity the numbers have to be unique.