Session arguments can be set by the FAXOPT function and to check results of the SendFax() application.
filename2- TIFF file to send as a FAX.
d- Enable FAX debugging.
f- Allow audio fallback FAX transfer on T.38 capable channels.
s- Send progress Manager events (overrides statusevents setting in res_fax.conf).
z- Initiate a T.38 reinvite on the channel if the remote end does not.