This is a fix to allows sending to non T.38 Faxs (older machines) 

Set codec in VoIP trunks -> advanced settings to PCMA (G711A)

under Fax/T.38 -> settings set the maximum transfer rate to 9600bps (non-Super G3)

G729 faxing will not work will older fax machines.T.38 is not accepted, so the call will be invited to G711A which supports all fax machines.