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)