I believe in many states, it's fine as long as at least one party to the call is aware of the recording. This protects people from unknown 3rd parties recording you. It can be a hairy issue when cross-border calls are recorded, and the states involved have different laws around "one-party consent". Read below...
http://legallad.quickanddirtytips.com/the-legality-of-recording-conversations.aspx "Once a court determines that a conversation is private, and covered by the statute, then it will consider various exceptions. The most common, and most litigated, is consent. Federal law allows recording of phone calls and other electronic communications with the consent of at least one party to the call. Thirty-eight states and the District of Columbia have followed federal law and permit individuals to record conversations to which they are a party without informing the other parties that they are doing so. These laws are referred to as "one-party consent" statutes, and as long as you are a party to the conversation, it is legal for you to record it."
its illegal unless the other party is informed that you are recording.
Maybe.
Many states are "1 Party States" meaning that if at least 1 person on the call is aware that it is being recorded it is perfectly legal. The state I am is that way (South Carolina). You need to check your state laws. To complicate it, some states allow you to use the law of either state (in the case of an interstate call) and some mandate you adhere to their law. Again, it differs from state to state.