In a "one-party" state, as long as one side authorizes the recording of the conversation, then you are good to go. If you ASK to authorize the recording and they deny it, then you lose that legal ground...
In a "two-party" state, both sides need to agree to record the conversation.
Look here for the individule state laws. If the call is interstate, the state with the strictest law will probably the best to comply with.
