I wouldn't try ironing normal transparencies onto the PCB. There are special transparencies for laser printers which can stand some heat, but I doubt that there will be deep dark toner on the PCB after ironing.
If the fax machine uses heat sensitive paper there won't be anything on the transparency.
For better results I suggest you use the special paper from PulsarProFx.com. It is coated with starch. The starch disolves in water and released the toner 100%. It comes in packages of ten sheets for $15. You can make a lot of boards froma single sheet.