It’s a USC title 18 violation and is generally a felony.
http://www4.law.cornell.edu/uscode/18/2511.html
I’m not a lawyer though, so don’t take my word for it!
Some states require notice to the second party, but many (most?) do not.
http://www.callcorder.com/phone-recording-law-america.htm
The U.S. federal law allows recording of phone calls and other electronic communications with the consent of at least one party to the call. A majority of the states and territories have adopted wiretapping statutes based on the federal law, although most have also extended the law to cover in-person conversations. 38 states and the D.C. permit recording telephone conversations to which they are a party without informing the other parties that they are doing so.
12 states require, under most circumstances, the consent of all parties to a conversation. Those jurisdictions are California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania and Washington.
It is illegal under all jurisdictions to record calls in which one is not a party.
A complete state-by-state set of regulations regarding telephone call recording may be obtained in the following report published by The Reporters Committee for Freedom of the Press:
http://www.rcfp.org/taping/states.html
(Google is a wonderful thing: “south carolina phone call recording law”)