I'm sorry that you're going through this. Yes, you can have witnesses. Yes, you can have her on speaker phone. Keep track of when she makes false calls, including the phone numbers so that you have it for your records.
I would simply add to Ms. Jones comments that you it would be best to inform your former spouse that someone else is listening to the phone conversation.
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You can record the calls if she knows about it: many cell phones have this capability. I would also jeep a log of all calls; date, time, and content.
You can also insist on email: text messages are a problem as you cannot get the content from your cell phone provider without a court order.