[Aleksandr]

The same problem can be solved through the mail server, if 3CX broadcast service information in the mail server - extension number and mailing address. And now 3CX fax server sends just such proprietary information:
Return-path: <fax@pax.ru>
Received: from ATC-3CX by http://www.pax.ru (MDaemon PRO v10.0.2)
with ESMTP id md50000007046.msg
for <fax@pax.ru>; Wed, 24 Mar 2010 00:15:00 0300
Authentication-Results: http://www.pax.ru
auth = pass smtp.mail = fax@pax.ru
X-Spam-Processed: http://www.pax.ru, Wed, 24 Mar 2010 00:15:00 0300
(Not processed: message from trusted or authenticated source)
X-Authenticated-Sender: fax@pax.ru
X-Rcpt-To: fax@pax.ru
X-MDRcpt-To: fax@pax.ru
X-MDRemoteIP: 192.168.0.15
X-Return-Path: fax@pax.ru
X-Envelope-From: fax@pax.ru
X-MDaemon-Deliver-To: fax@pax.ru
From: 33666 <fax@pax.ru>
To: Fax Delivery <fax@pax.ru>
Subject: Fax From: NO FAX-ID
Date: Wed, 24 Mar 2010 00:15:37 0300
X-Mailer: 3CXFaxServer
MIME-Version: 1.0
Content-type: text / html; charset = US-ASCII

Now 3CXFaxServer sends the port number and of the incoming email default address for faxes.
So give this idea to the developers. Transfer of service data, internal number and email of this number, from which was made to redirect the fax. And mail server parse these messages as necessary.