It's not illegal, but it's rather silly and will probably only antagonize the loon. I don't see this whistling stopping menacing behavior. Call the non-emergency number of your local police department and report the calls. Explain that you believe it is the person you have the order against. I assume you can not tell what the number is that they're calling from, but the cops may be able to get it from the phone company. You can also see if a local lawyer will send a cease and desist letter to the person--most will want at least a good faith basis that this is the person doing it.
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Do not blow a whistle. It will serve no purpose. Just do not answer the phone or answer and immediately hang up. If you have a lawyer, have the lawyer subpoena the person's phone records and if there is evidence that this person has been calling you, bring it to the attention of the state's attorney as that would be a violation of your order of protection. You might also call the police to report telephone harassment.
File a police report for telephone harassment and if the police can confirm the caller is someone violation an order of protection then those charges can be brought against that person too. It has been my experience that engaging someone who is harassing only escalates the behavior when what you want is to make it stop. Points for creativity on this one, but my advice is to personally ignore it and allow the police to investigate further.
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