The entry is filled by the data from the receiving end during the acknowledgement phase of the fax communication.
So it Should be the remote stations phone number, or their company name typically.
But that is from back yonder when a fax was hooked up to a phone line.
In this case I would venture that the remote station is not a fax machine at all, rather, a server running fax software, or possibly actual fax machines that roll over to another when one line is busy.
In the case of your MFP, if you log into the device and go to Machine Status > Tools > Service Settings > Embedded Fax Settings > Line 1 setup you will see the 2 fields pertaining to the number and name of your device. It could be that on theirs this is not setup, or not setup properly, or set up using a standard too dissimilar to the one Xerox uses.
I'm not sure what happens in the case of VOIP or FOIP machines, I honestly typically forget that fax even exists anymore :-(
If you could possibly confirm the remote device you are sending to is in fact an actual fax at an actual POTS phone line and has its fax number actually setup on it per the manufacturers method, come back and give me a model and I can see if I can replicate a similar setup in house.
That was very likely the most words I have ever used to say "I dunno, maybe they replaced hardware with software"