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.