I could not find any information related with your message on line faxing. Nevertheless this message is common on fax forwarding setups.
On those cases, the most common reasons are the device HostName which is not setup correctly or on recognizable way.
It cross my mind for you to check this configuration and additionally make sure that you have all the details correctly setup on the fax recipient device. Correctly setup the fax location setting as well as ID settings such as originating number.
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