One of the few things sketchier then bulk faxing its self is trying to save on bulk faxing. Awhile back I acquired a list of fax numbers for Canadian business and figured I might as well give it a shot. Being the cheap person that I am, I decided to look into trying to do my bulk faxing with VoIP. The proper way to do it involves T.38, however almost every single provider that claimed to support T.38 later turned around a few days latter after my account got transferred from the sales group to the technical group who knew damn well they didn't support T.38. Instead, what I ended up doing was setting up spandsp and using txfax over a high quality connection. For the most part it actually worked (in the sense that the test faxes I sent got through, save one or two). However, the response rate was horrible, so I gave up on it a few months ago.
More recently , I've been trying to get it to work again. However, a number of things have changed since last time. For one, it would appear that the VoIP provider I used has gone far down hill in terms of quality and I can't actually get a fax through at all. However, a number of providers are now claiming to support t.38 (if they actually do is another matter), so that might be a possibilty. In the mean time, I'm using a sketchy company in Ottawa (protus) to do this for me instead. There prices are higher, but they actually work right now.