Some printers still have dial-up modems in them, so can be used as Fax machines.
Otherwise you'd either need a dial-up modem (that's how fax's speak to each other .. you remember the noise right

), or use an online service that will send a fax for you.
Most
broadband modem/routers these days, though they may still use the phone line use a completely different signal frequency and technology that is isolated from the audible band .. so an old Fax machine wouldn't "hear" it and even if it could they'd be talking different languages.
BTW, who the heck is still using fax
