That's why it helps to have a script. Keep it by the phone, even (as long as you don't have one of those scary Tom Scola video-stream computer phones), so you don't have to worry about what you're going to say. Just read it.
Jot down a list of disengaging/redirecting topics, even, so if you get numb and tongue-tangled you can just glance at the list and tell her what you had for lunch yesterday or what recruiter just emailed you or what a pretty sunset you had last night, you wish she could have seen it. Anything loving and filial and not related to marriage at all.
And it sucks, it sucks to do it. Bleah. I spent so many years being frozen and frustrated and tearful with my dad (over different hot buttons and sore spots, but the same stuck misery), and I can say from grim experience that a script helps.
Also, remember that your invisible internet friends are hugging you and cheering you on all the time.
eta in response to your eta: Ugh, that joke thing is gut-curdling. I've never had the nerve to do this, but what my mom did, exactly once, with her own mother, was say pleasantly, "Mom, I love and respect you and do my best to honor you, and I know you love me but right now I feel like if you can say that, even as a joke, you don't respect or honor me. I love you, but I'm hanging up, and we can talk again some other time about something else. Bye." And then she quietly hung up, and curled up in a ball in the corner and cried because she was certain she'd just stabbed her mother in the heart and killed her dead.
But her mom survived, and lived to apologize, and never touched that particular hot button again.
YMomMV, unfortunately.