Incoming calls from PSTN get to AA on Unity. Then the caller dials an extension number, if the extension's CFA is set to VM. The call gets fast busy. Occasionally, the call can get to the subscriber's personal greeting. When I set the forwarding mode to consult PBX instead of release to PBX on individual subscriber's call forward page, this problem is gone. But the calling number becomes unity's port number rather than the dialing number, if CFA is not VM.
