My favourite example to illustrate the point, is to talk about my voice over work. When I record, I'm in a booth by myself. I don't just stand there. I embody the words and convey the message with my whole being. Only my voice will be used, so what I look like is irrelevant. But I do know that if I just stood still my voice would sound different.
My brother said something to me the other day which was the perfect example. He was talking about a pretty tricky phone conversation and at one point he told me he could "hear the other person raising their eyebrows". This phrase gave me a very clear picture about what was happening in the phone call, even if I didn't have all the details about what was being said. My brother couldn't really hear the other person's eyebrows being raised - I can't imagine anyone having eyebrows that noisy - but he could hear their tone of voice. It's highly likely that the person to whom my brother was speaking was actually raising their eyebrows and this is what influenced their tone of voice.
Next time you're speaking on the telephone, check in with the picture you have in your mind. Most of the time, we "see" this picture unconsciously as part of our interpretation of the message we are receiving.
You might even go a step further and try this out with a friend or colleague: ask them to adopt different postures (eg slump, stand up straight, smile, frown, check their Facebook page) and then speak. See if you can describe their body language. I've noticed that people are usually pretty good at deciphering the visual channel of face to face communication, even when it's invisible.
What are you conveying when you speak on the telephone? Check your body language. You'll be sending a message loud and clear; just make sure it's the one you intend to send.