Using OBi110 with low-cost Localphone VoIP, on my Windows PC, FAX Send & Receive now work reliably.
The trick is, in Windows lower the "Maximum Port Speed" and reduce the number of "Receive Buffers" & "Transmit Buffers" until FAX transmissions become reliable. The optimum settings are hardware and VoIP dependent.
Reference:
http://yourbusiness.azcentral.com/change-fax-transmission-speed-windows-xp-21403.html (http://yourbusiness.azcentral.com/change-fax-transmission-speed-windows-xp-21403.html)
Hope this helps someone else.
This method also works for me with Callcentric. My PC just transmitted a 12 page FAX using Callcentric; it went through, without any errors, on the first try.
:)
Be nice if carriers supported T.37 (http://en.wikipedia.org/wiki/T.37) and a capability to connect two T.38 (http://en.wikipedia.org/wiki/T.38) aware endpoints (no ATA).
Some light reading. (http://www.soft-switch.org/foip.html)