ALL parties to a conversation must know that the recording is being made before it can be used in court (Of course, that does not apply to recordings made pursuant to a legally obtained warrant).
In other states only one party to the conversation has to know about it.
I believe that in conversations involving people in multiple states, a person in a one party rule state may record the call, but the recording cannot be used as evidence in a legal action in a two party rule state's courts.
Answering machine tapes are simple. You obviously know you're being recorded.
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