Last year, we switched to Optimum Voice through our cable company. I am still using the same fax machine that I have had for years. NO problem with it. Can't be of any more help than that, though, because my DH took care of the technical end of it!!
I have never heard of being able to use a fax machine with a cell phone,though, but wouldn't doubt we will be able to do that too at some point, if not available now.
Lorie