I use the windows version of FreeSWITCH with fax modems on a Windows Server box, running the normal Microsoft Fax Service. It's been working using the SPANDSP fax modems perfict for over a year.
 
The thing you have to remember is that the modem in mod_spandsp is T.31, not T.38. If you wan't T.38 you need to run the call through T.38 Gateway, just as if the modem device as a TDM line. Look at the SPANDSP wiki page for reference. I know this works on windows, and should also work on linux.
 
I'm not sure about the built in client fax in windows 7, but I know it works with the fax service role on windows server.
 
--Dave

From: Steven Schoch [mailto:schoch+freeswitch.org@xwin32.com]
To: FreeSWITCH Users Help [mailto:freeswitch-users@lists.freeswitch.org]
Sent: Fri, 18 Jan 2013 17:40:00 -0800
Subject: Re: [Freeswitch-users] FAX problems

On Tue, Jan 15, 2013 at 6:52 PM, Dave R. Kompel <drk@drkngs.net> wrote:
Another option would be on the Linux box, enable SPANDSP modems, and use com0com on windows to backup com ports to raw TCP connections. It supports both. That way it would talk over a TCP pipe to an emulated modem on your other FreeSWITCH box.

I just tried that with limited success.  I turned on 2 of the SPANDSP modems (and figured I had to give the freeswitch account (the user that runs freeswitch) write permission to the /dev directory so it could create the symlinks).

I then installed com0com on my Windows 7 box, installed sercd on the FS server (Linux), and ran hub4com on the Windows box to connect to sercd.

As a result of all that, I can use Putty to connect do the CNCA0 serial device, and issue AT commands, and it responds like a modem!  I issued an ATDT <fax number> command and the call came through to the fax machine.  (This is the FAX machine connected to the POTS line.)

However, when I installed a "Standard 56000 bps Modem" connected to CNCA0, and attempted to use it to send a FAX from Windows, it said "An unexpected problem occurred."

To make sure Freeswitch can actually do FAX, I ran
originate sofia/gateway/<gateway name>/<phone number> &txfax(/path_to_fax_file)
where the fax file specified was a TIFF file that I had lying around.  This was successful.

I'm now considering installing the Windows version of FreeSwitch on the machine of everybody who needs to send FAXes.  That may take less time and trouble.

-- 
Steve