Federal Law:
it is permissible to intercept and record a telephone conversation if one or both of the parties to the call consents. Consent means authorization by only one participant in the call; single-party consent is provided for by specific statutory exemption under federal law. 18 U.S.C. Sec. 2511(2)(d).
Link to every state's laws:
"Can We Tape?"
As you will notice, most say exactly what I said, that ONE party has to know. And that one party can be YOU, the recorder. Only 12 out of 50 states do not allow taping with only one party's consent.