Is it legal to tape a phone conversation in Idaho of a person you are speaking with in Minnesota?

I have a feeling that an employee of a previous employer is slandering my reputation when recent potential employers have called about my references. I am considering hiring someone to call as if they are a potential employer to verify that I was employed by this previous company and to inquire about my work history. But I am in Idaho, and the previous employer is in Minnesota. Are there any specific laws between the 2 states that would render this inadmissible in court? If this person is saying what I think she is saying, I have full intentions of taking her and possibly the company to court for defamation.