"People reasonably expect their telephone conversations to be private whether made from home or a public telephone booth. Police need a search warrant before recording or listening in to telephone conversations. Federal laws enacted in 1996 extend the general privacy to electronic devices like cell phones and e-mail.
[However,] under the Patriot Act as reenacted in 2006, the National Security Administration claims the power to listen in to private conversations that may involve domestic terrorism. Any phone call placed to or received from a non-U.S. exchange is currently subject to a warrantless search. How widespread the domestic surveillance is, and the legality of such surveillance, is a subject of furious debate." -From, "The Criminal Law Handbook"