Hi Sarah
sorry about the diversions. You'll have discovered that caring is an activity that is draining and has lots of obstacles and challenges. One of the ways we cope is to bring in the surreal. Doesn't mean we don't care, just means we are all trying to keep on top of it.
About the short phone calls, well I'm not sure there is an easy answer. When a person is with you, you can use distractions of many sorts to continue a conversation, no matter how tenuous. In such a situation, you can walk, talk, gesticulate, pull faces, listen, touch, etc.
But on the phone, one half of the conversation is out of your control, and if they put down the phone, that's it.
I've not met this particular problem, so I can't speak from experience. All I can suggest is that, each time you phone, you try something different to see if it works, then stick to it.
Try varying the loudness and tone of your voice, try different topics, try....well, try anything. And if it doesn't work the first time, try it again.
But for the moment, as Chesca says, make the very most of seeing her in person.
Very best wishes