I understand that it may be prohibited due to the laws of "Dina D'malchusa Dina," (when and if) American law prohibits it. However, that reason aside, is there any Judaism based reason to prohibit it?
From a Halachic standpoint I do not see why it would be Assur. Why is it any worse than a guy approaching you and trying to sell you something. Today with caller ID you can ignore the call if you want to. In addition you can always hang up on the guy, which is a lot harder to do to someone who approaches you in person.
What I think is Assur is to fax someone faxes without his permission where then you are using his ink and paper and he can't do anything about it.