How would you react if your child answered their grandmother's call, she hung up, called again, and…

How would you react if your child answered their grandmother's call, she hung up, called again, and they answered again? And she started accusing them of not wanting to talk to her, of being a liar and naughty. Background: the phone was on the charger, and while it's charging, no one in our family uses phones. My mother was sent away, of course, because my patience isn't limitless. The situation is like a textbook narcissist case – denial, as if the attack never happened, as if I made it up myself. And the child, after such attacks, doesn't want to talk to her. I never interfered in their relationship: I didn't project my not-so-great relationship with her onto my son's relationship with her, I never said anything bad about her. Even when my son asked: "Why doesn't Grandma like you?". Although there's plenty to tell.