I am an awkward phone conversationalist. I attribute this to the fact that I am very visually oriented. I learn things much more easily if the content is presented in a visual format. In conversations, I depend on reading body language cues from the person I'm talking with, and I feel more confident knowing I can rely on gestures to aid in my own expression or clarification. I can help myself hold a better telephone conversation with preparation and notes. (This sounds silly, even to myself, because of course preparation should help anyone. Recognizing it and acting on it is a good practice.) But I often wish I was better at off-the-cuff discussions.
Other exercises have helped me improve this skill. Offering verbal critiques of theatrical work--especially immediately after viewing--as I had to do throughout grad school, was excellent practice. Writing is also a beneficial exercise, as it helps me refine the act of structuring my thoughts, and offers me continuous practice in using elegant grammar. Choosing how to express an idea--deciding how to articulate a thought--forces me to clarify what I really think about the matter.