I think one of the reasons we all get flustered over the phone is psychological. When you're speaking to a person face to face, you can see them, you can make eye contact, and (awful as it may sound) you can see their flaws- which in turn makes them a REAL person, a person who f***s up just like us, a person who you could charm with a smile, or wit or your bubbly personality. Put them on the other end of a phone and they're just a voice. In our minds eye they may look like the perfect person, be infallible in every way, ultra professional and may as well be a superbeing/robot!!! Plus- you know YOUR weaknesses and insecurities and the more you talk the more unsure you are of yourself. It's a common feeling that most people have...try to stay calm by imagining them naked on the other end of the phone (or even doing something very normal like going to the toilet or farting- I'm sure they do it too after all they are real people just like you!). It may sound absurd and make you laugh but it really does help to see them as simply another human being! :)