We are having inconsistent outgoing FAX results on both SIP and PRI trunks. Incoming works fine every time. Sometimes outgoing will fail on the first attempt and sometimes will work every time to specific FAX machines and not at all to others. We think this is maybe due to SG3 fax protocol used. It may have to do with the selection of the correct CODEC on outgoing FAX. How do we specify CODEC on outgoing?
Do we need separate dial peers for fax and regular voice calls?
Simple solution may be to throttle the speed on your fax machine. You should be able to set the baud rate the 14,400 or below. You can also try turning off ECM if your fax machine allows it.
How is the fax connected? FXS port? In the past there was a bug with SCCP controlled FXS ports with a fax machine connected. If the above does not work, you could configure the FXS port to use H.323 instead of SCCP.