depends on the fax machine. You don't have to program the fax # in.
Plus it could have been on there and removed after a few passes.
I was able to get close to the same effect with a couple stock photoshop filters. If I had time to run it through a bad copier several times, I could have nailed it even better.
My machine will generate a report of ALL incoming faxes for the time period I specify showing the originating fax number. Maybe that could be done on the machine that received the info.