What does Priya understand at the end of the story?

Every Sunday at six, Nani's phone rang, and every Sunday it rang for a long time before anyone answered. By the time she had found her glasses, found the phone and found the green button, the call had usually stopped.

"Just press it, Nani," Priya said, for perhaps the hundredth time. "Green is yes. Red is no. That's all."

Nani looked at the phone the way she might have looked at a snake. "Your uncle's face comes into it," she said. "How does his face come into it? He is in Dubai."

Priya opened her mouth to explain, and then closed it. She had been about to say something about signals and satellites, but she realised she did not really know either. She only knew which button to press.

So she tried a different way. She sat beside Nani and pressed nothing at all. When the phone rang at six, she waited. Nani's hand hovered over the screen, pulled back, came forward again, and pressed green. Her son's face filled the screen, laughing.

"You answered it yourself!" he said.

Nani did not look at him. She looked at Priya, and for the first time Priya saw that her grandmother had not been slow. She had been afraid.