Fax and cellphone do not go together. Basically, the way a cell phone handles the transmission is incompatible (to keep it simple).
Now, having said that, if you had a fax machine that was designed not to send out a FAX signal, but instead sent out a digital signal that is compatible with VOIP, you might be able to use the hotspot feature of your phone and send the fax over VOIP.
It's not simple to do and, to be honest, I am not very confident that it would work very well.
Have you considered using an Internet FAX service? With those, you can use a scanner (or All-in-one printer) to scan the pages into a computer and then send them to a fax machine.
Some use a printer driver to do it. Some use a special piece of software and some of them have you upload the files online to a web site and send it out from there.
I have used a free service from Myfax that works well. If you need to send a lot of faxes, they have paid service too. I also have used EFax, which works pretty well. Another is called SFax. I just started using that one recently and it works pretty well too.
So, you do have some options available. Like I said, the VOIP fax might work but I have never had much success using that over a regular VOIP line on broadband, nevermind trying to do it over the cell phone's data link (which is very bursty in the way it works).
OK. My 2c.
