I unhooked myself from the outside world at the box, and now have 3 phones (one line) plugged in; you can have as many jacks as you want running simultaneously if your house is wired correctly. I think you can get a second line for $10/month, so your fax will work fine, I think.
I do have a problem - signed up with Vonage when I moved back into my house (didn't want to pay Bell South $100 for hookup, so saved there), the quality on some international calls seems to be a bit faint - anyone had that problem? Of course, at 80% off for Intl Calls I have no complaints, but have to pick up the phone to speak (speakerphone not loud enough).
Overall, great service, though.