Faxing is a point-to-point modem connection directly between two fax devices. It absolutely must involve the telephone network. (Even if you don't have broadband, you aren't connecting to the Internet to fax, you are dialing the modem directly to the recipient fax machine.)
If you don't have a land line, your only option really is an internet fax service like eFax.com. They don't have the same level of Mac support as they do for Windows, of course (so no faxing from the Print dialog), but you can email a message to be faxed, or upload one of the many supported file types to their website, and fax it from there. eFax is not free to send. (eFax works similarly to the IP relay services, except that instead of a voice call being placed, it places a fax call.)
tooki