I tried faxing over my Vonage line today via the Motorola VT1005 gateway with my Brother M740 fax machine. Nada. Just a click and dead.
So I called Vonage and the dude was awesome. Raised my voice quality back to highest (now need to check my router again), changed my SIP registration expired down to 300 and had me dial with *99, to tell the moto it is a data call.
It worked flawlessly on the first shot!