Good idea. I have received numerous fax calls from some convention booking service which seemed to have entered my number by mistake (the real number was one digit different from mine). And those calls came in the middle of the night! With your "Do not call" reply, that would have solved the problem.

One possible way to implement this, would be as a service at the local phone provider's switch. It could be a service ala caller-id blocking, whereby the switch would be programmed to reject fax signals for a subscriber's phone #.

Frankly, though, I don't think that the demand is great enough. I cannot remember when last I had a fax signal blast my ear drums on my phone line.