Illegal harassment by Alliance Interstate LLC?

Forewarning: my explanation is lengthy! My question and how to deal with this problem is closer to the end of this post.

My mother-in-law was contacted yesterday by Alliance Interstate LLC. However, it was not her that that they were interested in, but my father.

When she answered the phone she was asked if she was my father, to which she replied in the negative, obviously. Then she kindly informed them that she knows him, but that he doesn't live at her residence. She proceeded to ask who it was that she was talking to and what they needed. She was responded to in a blatantly rude manner and was told that they couldn't tell her anything about John Smith (not my father's real name). She said their tone and the way that they were talking to her was just plain mean. The cherry that topped the entire conversation was with them hanging up on her.

She knew full-well that this was a collection agency, because she's been receiving these harassing phone calls from them since I married into her family.

Unfortunately, two of my close family members (my father and sister) are riddled with debt from bad decisions. Even more unfortunate is that my in-laws - who maybe see my father once a year, at best - have been harassed off and on over the years with these phone calls. They've been patient in dealing with them, but yesterday my mother-in-law was very upset at the rude behavior to which she was subjected by crass individuals on the other end of the line.

In my opinion, this is nothing short of harassment.

I would like to know what legal options are at my disposal to stop these kind of harassing calls.

Thanks in advance for the help!