Thanks for posting. I noticed your technical support ticket reference this, so I thought we would share the same response here. We would recommend connecting the fax machines to the FXS ports on the 6355. You would then set the trunk-number on the FXO trunks to the voice users configured for the fax machines and reject those numbers on the grouped-trunk for the SIP trunk. This should allow you to route inbound calls to the fax, but use the FXO for outbound calls in the failover scenario.