Hi AnonLeeds,
Welcome to the site

For me understanding BPD behaviors has been really helpful. I am able to take things less personally when I understand what is going on.
The conversations regularly seem to revolve around “John” blaming his mother for all his problems, saying she needs help, that she abuses him verbally and is totally disrespectful to him.
The above stood out to me. This sounds like a lot of projection...negative feelings about SS(stepson) that he can't handle so he dumps it on your wife so he feels better.
More information on Projection...
https://bpdfamily.com/message_board/index.php?topic=70931.0While some conversations between “John” and his mother remain calm, they frequently end with “John” shouting and verbally abusing his mother. She has recently refused to see him on her own after a particular incident when she feared he might get physical.
She is setting a boundary here regarding her physical safety that is a good thing. Has she ever tried setting boundaries around the abusive phone calls?
She also wants me to listen to their phone calls.
I would be wary here, why does she want you to listen? Does she want to problem solve or does she want you to take her side in the conversation? This could lead to Triangulation so it might be something to avoid.
While I disagree with the above, what I do recognise is that my wife does not acknowledge his feelings or change her response to what he says. It’s the same battle between them almost every time and I am more and more in the middle trying to observe and counsel my wife.
It sounds like they are stuck in the same dysfunctional dance they have always done, when you're in it, it's harder to see, and when you've done it for years it's hard to undo that conditioning. You are able to recognize areas for improvement because you are on the outside looking in. That is the part of the step-parent role...that outside perspective.
It's good that you are recognizing you are in the middle, because that isn't always a good place to be. More on the Karpman Triangle...
https://bpdfamily.com/message_board/index.php?topic=108440.0It's great that you want to help, but one of the things I learned early on was there is only one person I control and that's me. I couldn't control my Partners undiagnosed BPD ex-wife (uBPDxw), my Partner, or their 2 daughters. I arrived here an angry/frustrated Panda because I was seeing all kinds of things I wanted to fix or protect them from, but they all were in their usual roles doing the same dysfunctional dance.
I finally had to let go of trying to control, I could nudge,I could make suggestions, I could provide support, I could provide information, I could change what I did, but I couldn't do it for any of them. I say all of this not to discourage you from giving your perspective/ideas/support but keep tabs on yourself because you can take on too much (things that aren't yours to take on) and become resentful.
Would your wife consider joining our site? My Partner and I are both members and it really helped us to speak the same language back in the beginning.
Again Welcome

Panda39