My understanding of FAX over VOIP is that it is really hit-or-miss. Some people have it working while others can't. I tried using my old Faxmodem on my VOIPo line and couldn't get it to work. I tried changing the settings and even adding a backwards DSL filter to no avail.

VOIPo give you free outgoing fax on your vPanel. Works great for me. As you said there are some free incoming fax solutions on the 'Net or you can get a $36/year fax number from VOIPo that will forward incoming faxes to your email as .PDF files.

If you really need to use a "fax machine" for high volume work or whatever I suggest you keep a landline for that. Use VOIPo for your voice calls.