FAX can only be connected to the fxs, not fxo. Also fax on ramp is not supported on the fxs port. This means the following fax flow is not supported.
FAX--connects to --fxs gw fax on ramp to mail server
OR
FAX---psdn-----fxo gw fax on ramp to mail server.
fxs connects to a port right after it detects the loop by the fax