It depends on the jurisdiction, some states permit recorded conversations to be used as evidence in a proceeding where only one party was aware of the recording, and others require that both parties be aware of the recording for it to be admissible. I'm not aware of laws that make the act of recording the conversation illegal, however, just whether it can be used as evidence or not. There might be, but I don't know.
It is a state-by-state issue -- some states only require one party to consent, while others require two. Information on this is pretty easy to find. But you'll also need to consider whether you're calling someone over state lines, because then you're not just invoking state law but federal as well (though I believe that's a one-party consent rule, anyway, http://www.law.cornell.edu/uscode/text/18/2511).