What evidence do I need to prove Defamation of Character?

The story that you described is probably best described as a "Manifest Injustice" through trickery and entrapment. You need to make the voice mail available to the court. I cannot imagine how this is done without a lawyer's assistance. Regardless, it will not excuse your violation of court orders. Your son's mother's statements don't overrule the order of the court. As for the Defamation of Character. Defamation of Character is an untrue statement made (public) that harms you in particular ways. The harm may be by reputation, presumed, economic or physical. Proving her dishonesty may be an insurmountable task. I also presume you have not HIRED your own lawyer to represent you. Well, good luck.