I strongly suspect that there may be problems in the marriage of your father and step mother, and that is why he keeps calling your mother. A lot of men are emotionally infantile (a lot of women too ) and cannot function without a woman's input.  So when they divorce they go onto the next partner very quickly if they are not already waiting in the sidelines.  Men tend to do this more than women, who tend to have stronger female friend networks. 

I would strongly urge your mother to just tell her exH that his calls are not welcome.  Having told him, she should not pick up any more, let his calls go to voicemail.  If he gets no reinforcement he will stop.  

Unless you think your dad will get physical, let them deal with it

Stay out of it, they are grown-ups. Your mother shouldn't be putting you in the middle. Why is she even telling you this?

Is your mother incapacitated in some way and needs an advocate to advocate on her behalf telling dad to stop calling?

I thought it was very strange that YOU have to scold your dad not to call her and be one way or the other with his wife.

It looks like these BMs we are reading on here about who get their kids involved in the middle of their drama with their exes. I understand you can’t control your parents. I have insane dad myself but you can always tell them that their relationship isn’t your business.

I can’t imagine my DD telling me or exDH how often we should talk to each other or how we should treat our spouses. And my DD is very outspoken and opinionated. But this is  so not her business! 

Stay out of it

We don’t really know what he is doing to his wife that is horrible. When ex and I have to talk to each other about something it’s not any secret from our spouses

If he is calling with romantic propositions it’s terrible but if mom doesn’t like it she doesn’t have to pick up a phone af all. Or is she thinking it’s romantic? Or for whatever reason she likes to talk to him and now create drama between all parties  and get her daughter involved? If she is uncomfortable why is she answering? 

bottom line it’s between them 100%

While my parents were divorcing they both tried to recruit me to their "side" and use me basically as a dumping ground for their frustrations. I was sympathetic, they are my parents. But there was a line and they crossed it. Their marriage is not my marriage or my problem. It is not my job to solve it, mediate it, or even listen to it. And  I told them both that. If they talked to me about it I'd start sending them bills for my time. 

I had to do it with my dad and my brother.

And I'm about to have the "not my problem" talk with my mother and her sister. 

Not your monkeys, not your circus. 

My mom did this to me about my alcoholic dad starting when I was a child. I was her Emotional Partner listening to her problems & fears (= my dad) and her Inverse Parent as she sought my council & support (rather than a peer or a counselor). 

After 20 years of mostly No Contact, I now communicate with my parents. However, my mom is on notice that access to me stops if she tries to ask my advice about her marriage. I literally sat in on a counseling session my mom invited me to before I shut off communication, & her counselor within the 1st 5 minutes told her that even if she was 100 & I was 74, it would still be inappropriate to dump her emotional & marital baggage on her daughter, and that it was wrong to have burdened little StepUltimate with that as a child. 

My parents are still "together," BTW.