If they don't have a fax number, then they can't get email-fax. To extend this for people who don't have fax numbers, we could make up some phoney phone numbers for TPC.INT that would be mneumonic, I suppose. But you could just register some other number that wasn't really a fax in tpc.int, too.
Someone has to know how to map numbers into email addresses.
You could shortcut it with a directory service, I suppose. The scenarios seem different to me. Actually, no, I don't use JFAX's client software, it's not required. But it's true, you have to have a gateway that pretends to be a fax recipient even when it's not. If you want to send faxes from your laptop, this isn't different.