The call is being routed from the  Microsoft FAX plug in.  The routing works like this:

1. FAX is initiated with Microsoft FAX on Windows

2. It goes through the faxback.com plug-in on the same machine.

3. The plug-in is configured to make a SIP connection to the FS pbx.

4. The FS pbx routes the call to the PSTN through sip.flowroute.com.

5. The call is received by a POTS line, connected to a modem.

6. The modem is connected with a serial line to a Linux machine.

7. The Linux machine runs Hylafax (www.hylafax.org).

Steps 4-5 work for voice calls, using Polycom phones.

Steps 5-7 work for other FAX calls, received often.

The log for the faxback.com plug-in is at http://pastebin.freeswitch.org/20448.

The FS log is at http://pastebin.freeswitch.org/20449.

The Hylafax log is at http://pastebin.freeswitch.org/20450.

New info:  I put a test set on the PSTN fax line.  (Yes, I'm old school telephony.)  I hear the fax modem giving the normal terminating fax pre-message signal, and the originating fax sending the normal CNG tones.  However, these tones don't stop.  It's like the originating fax can't hear the pre-message signal from the fax modem.

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Steve