In Washington, audio recording of a conversation without the knowledge and consent of the other party is a crime. Voicemail, on the other hand, is fair game, because our courts have held that a voicemail has no expectation of privacy.
You can always recount for a court what someone said to you. I advise my clients not to use really vulgar language, no matter how accurate it is, but instead to say, "and then he used really vulgar language about me and my mother" or something like that.
Proof of verbal abuse can be made by you to the court. Don't risk the court's ire by breaking laws. Other attorneys are going to disagree with me, but this is what I think. Hope it helps.
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It is indeed a crime. And, it can be serious. I know of someone who was actually prosecuted for doing this. The issue of "verbal abuse" sounds as if it is part of a much bigger picture. I would suggest that you consider a discussion with your attorney about how best to both protect your children and make strategic use of the bad behavior of your spouse.