I attempted to call the number that the failed faxes were sent to and it is plays a message saying the number "non-working" and gives an alternate "888" number to call for assistance. I would suggest calling the old fax number from a voice line to get the assistance number and see if they have a new number to send the faxes to.
I tried to see why the best we could report was "Unknown Status" and log shows the call went from "ringing", to "answered", to "hangup" showing the remote end terminated the call normally with no indication it was voice or fax. After 5 attempts with the same result it gave up. In this case, there wasn't anything concrete enough to give a more informative status message.
Hope this helps!