These things have so many variables, so it is impossible to comment directly to your situation, but having worked in residential treatment and with childrenin foster care for 10 years, contact like this can sometimes be difficult, but ultimately very important. As another pp mentioned, they are vital sometimes in helping children process why the adoption happened as the child can come to see the discrepancies in their behavior as well, or in a "better case" scenario, can show that they are loved, even if the parent couldn't care for them. They also give the child opportunities (down the line, when they get a bit older) to ask the direct questions they need to ask to fulfill their own sense of peace (if they need that). There is sometimes a delicate balance between allowing the calls (and knowing they may be upsetting) and being the person to stop them (why didn't you let me talk to her?!). For my own experience and conclusion, it seems that barring truly manipulative or traumatic statements, IMO it's better to let it play out and help support the child as they figure it all out. Its not easy, but that is the hand that has been dealt. I had kids whose parents were GREAT phone parents and in a way, that was really confusing to them too- How could my mom be so great and normal and yet couldn't care for me? The kids that seemed the most stable and strong were the ones who could see that their parent loved them but the parent was unable to care for them. And, this often happened with *more* contact and not less, as the child could see for themselves what was there. Is it hard to watch? Youbetcha. But it was 10 times harder to watch the kids that grew up with these burried, unanswered questions or parents who they only had such minimal and emotionally charged contact that the few times they had contact it turned into total melt downs. The bumps and bruises of some inappropriate (but not traumatic) calls were so much easier to talk about and heal from than the "bomb drops" of the ones that were shut out.
Additionally, if you have the chance to talk to the parent yourself sometimes in a positive way, that can give you an opportunity to maybe present some other constructive options. Many of the parents of the children I worked with sincerely loved their children, but were absolutely unable to care for them, had very little sense of what positive parental roles were, and were coping with huge problems themselves and so the calls were often ways that the *parent* would be asking the *child* for validation, love, and support. Sad, but inappropriate. So, I'd try to catch the parent and gently suggest things like asking the child how he was doing in school, or asking them about the t-ball league or how they were going to spend the weekend. While to us this may seem obvious, sometimes parents can be detached and wounded and not know what to say to focus on the child. You can also maybe suggest some other ways the parent might be able to serve both their own pain of the events without putting it on the child... Some I've suggested to parents to write a journal to "give to the child when they are older" to tell them about their life, so instead of bombarding them with adult issues at every phone call out of the blue, that can go to "the journal". Things like this. I've also dropped suggestions of more frequent but less fueled things, like "Hey, send a Valentines card! Billy would love that!" (which is usually purchased at the store, has a cute pat phrase and the parent signs it and it is easy for everyone to be happy about) or "Send a picture of your new dog! Suzie loves animals." (which gives an opportunity to keep it light and child-friendly). You don't always want to be the intermediary, and you are not the parent's therapist, but if contact is happening, it may be something to think about as to how you may be able to make it the best it can be because that ultimately benefits your child.
OP- trust yourself to judge if these are upsetting but part of the learning of their own life story or if they are truly harmful and do what you can and/or need to do. And, of course, you have your own relationship with the mother and would need to work from that as well. My post (I hope!) is more of a "Well, this is what I've come to and have done..." but is not necessarily a specific response to YOU, as there is no way to know if this is possible or beneficial. But whatever happens, I wish you wisdom and peace .