Governor Walker ought to sue for invasion of privacy, and Mr Koch should sue for defamation of character. That would teach these lowlifes a lesson.
At some later date possibly. Doing that during this budget battle would only serve to draw the public's attention to a somewhat embarrassing phone call.
For now, it is probably best to laugh it off and correctly point out that there is really nothing damaging in this call. Walker says essentially the same thing on the phone that he does during a press conference or interview. Even when baited by the caller, he is calm and reasonably and doesn't say anything outrageous, illegal or otherwise unseemly.