legal rqmt for FAX return phone #

I thought I heard somewhere that it was illegal to send anonymous faxes -- namely, that there was some regulation that you had to program your FAX machine with your phone number so it would get sent automagically whenever you faxed anything. Essentially it was ILLEGAL to fax without providing the recipient with a mechanism to find out where it came from. Of course, I suspect enforcement is lax, but still... :-) :-( Can someone please confirm this, or help with a reference? Private e-mail is fine... -- dat@ebt.com (David Taffs)