Yes, that applies to wiretap laws. Everyone (varies with state as mentioned) has to be informed and accept that they are being recorded for the call to be admissible as evidence in a court of law. Without that permission, it will be discarded.

I should add; if this were not the case, then manufactures of modern gadgets would be much less likely to have active monitoring of your microphone, for fear of how that action could be interpreted in court. Siri, what are your thoughts on this?