I am not enjoying being with Virgin Mobile at all. First, I find out belatedly that you do not offer three way Conference calls. I would never have joined if I knew that from the very start!! Then I discover that I am having this same issue with Call Waiting just like the many others have described here. It seems to be a general problem with Virgin Mobile and has been happening for a while.
In an answer to a similar issue from 2018 a colleague of yours said they "had to change the Call Forward when Busy divert to OFF on the network side". Please look at my settings and see if that would resolve it for me otherwise I will leave as soon as my contract ends!