The vibe I get from your recent interaction is that your W might be extra sensitive to feeling engulfed/overwhelmed, and also is very low skilled when it comes to communicating that feeling.
For example, a "generally normal" person might, after 1 hour on the phone, be able to say "well, this was a good conversation, but I need some time to think about it before we go on much more".
Your W may have been feeling pretty good in the moment -- in each moment -- through the 3 hours, but at the end perhaps started to feel overwhelmed, and, having poor/low skills at communicating feelings, became abusive in order to get her needs met (i.e. used getting abusive as a way to end the conversation that maybe had started to feel overwhelming).
She may have still not come back to baseline the next day, may still have been feeling overwhelmed/engulfed, so even a very normal message like "thank you for talking to me last night" wasn't something she had the tools to cope with. So, again, she may have sent the toxic messages as a low-skill way of getting more space for herself.
That doesn't make it okay, it's more of a theory as to why it seems like things can go really well and then she gets so hurtful to you.
I'm wondering what it'd be like for you to, for now, take the lead on "quitting while you're ahead" or "quitting while you're winning". What I mean by that is -- three hours is a long phone conversation for anyone! What would it be like for you to find a way to gently end the phone call a lot sooner, while everyone is still pretty regulated and positive? She probably doesn't have the skills or awareness to do that, so it may be up to you to be the leader there.
That could also give both of you a different experience -- where it's not her abusiveness that dictates how long a conversation goes on, it's a positive ending.
Just a theory -- would be interested in your perspective.
I took some distance and replied with a voice message so she could hear that I was calm etc and it had no effect, suddenly abusive, calling me constantly and I told her I would call her tomorrow as she is clearly not in a good mood.
Did you end up making that call? How did it go?