How can I talk to my mother (who idle-chatters) without getting angry?
When we want to deal with someone, we need to first understand that someone. The more information or knowledge we have of that someone, the easier it gets.
Now in the case of your mother, isn't it obvious that she desires to stay on the phone as long as possible with you? She doesn't do the same when you're physically with her, because she's happy enough that you're already there. So what do you think is happening here?
As I see it, she wants you to be present. Talking to you on the phone allows her to feel that you are present. And what does she get with that? No much actually, except for short periods of relief, I guess, from loneliness. And why talk about unnecessary things? Why not? The subject obviously doesn't matter. Even if it's a repetition, whatever comes to her mind is a way to keep you with her, to keep her from feeling lonely.
You, as her child, wish to please her, and so you keep listening. While wanting to be nice, you listen with anger. Even if there's not much of that in the beginning, it builds up in you. And when you can't hold it anymore, you explode.
Frustrating, isn't it?
You might think that by listening quietly she might eventually recognize that you want to end the conversation and that solves your problem. Hasn't work so far, huh?
When we let someone with lots of desire lead a conversation, then it's to be expected that the conversation becomes dominated by desire. Since between you and her, you have a clearer head (i.e. more wisdom), and you want the conversion to be more meaningful, then you are the one who has to take the lead.
Stop trying to be nice, and start being kind instead, and courageous.
Help her. Help yourself. Lead the conversation. Choose the time to call her. Tell her how much time you have with her on the phone. Ask her about her day, her feelings. Don't let her defilements lead. Let your wisdom and love lead. Let her feel your presence, your love for her.
All the best to you.