If it is a velied threat as, you say, then I wouldn't waste much time with it in court, because it probably is not going to overcome the mediator's recommendation. And you only have maybe 5 minutes to speak so if you spend all your time on this recording you may not get anything important said. Rather, I suggest you seek a local attorney to help you prepare, or to go to court with you, to make your best arguments and increase your chances for sucess.
But if you must go to court without help from an an attorney then (1) Transcribe, (type up word for word), what the recording says. And write at the bottom of the transcription that you declare under penalty of perjury that you transcribed the call and that this is a true and accurate and complete transcription of what the voicenmail says, and sign and date it at th bottom of thd transcription. Immdediately serve a copy of the transcript to the other side -don't spend time worrying about deadlines just get it served as soon as you can to make it more likely the court will accept it, and file the transcript with thd court asap so the judge has a few days to read it BEFORE your next court hearing. If you just bring the transcript to court the day of hearing the judge probbaly will not take thd time to read it. Also, bring your cell phone to cout and have the message ready to play for the judge if the judge wants to listen, but you are more likely to have it considered by the judge if you submit a transcript as I just ex[plained, and a lawyer can help you with the trasnscript. The court might NOT allow it in since I assume you did not tell the othd party you were taping, but try it anyway - possibly mom won't know to object that she wasnt told of the taping. But again, don't waste too much of your precious 5 minutes in court with this phone call and have other GOOD arguments ready if the judge won't deal with the telephone message or if the judge doesn't care much about it.
Thomas Neil is a Sacramento attorney, representing clients in court in Sacramento and in surrounding counties. Or, if you cannot afford full representation then, for much less, Mr. Neil can instead write you the forms and declaration you need, and help you serve them, and you go to court by yourself. A well written declaration by an attorney, supported by proper evidence, will GREATLY increase your chances of sucess in court. Our office takes credit cards.
Thomas A Neil
3224 El camino Avenue
Sacramento, CA 95821
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