The ScanSnap is a scanner, not a fax. Without extra equipment, including a land line, you can't do that directly.
You best bet is to scan and then email to an online fax service, e.g. eFax, which will then forward it (at a price) to any dinosaur, er person, receiving the fax 😉
eFax (or others) can also have return faxes sent to you via email.